With regular custodian Scott Carson out injured, Roos finally made his league debut for Derby on 19 January 2019, helping the “Rams” to 2–1 victory over Reading. Manager Frank Lampard said he then faced a “nice but difficult” selection decision between Roos and the returning Carson. He stuck with Roos until the end of the 2018–19 season, as Derby went on to reach the play-off final. Derby were beaten 2–1 by Aston Villa in the final, with Roos being at fault for the second Villa goal after misjudging a shot from Anwar El Ghazi to allow John McGinn to head the ball into an empty net. He signed a new three-year contract with Derby County at the end of the 2018–19 season.

Following a number of poor appearances, manager Phillip Cocu dropped Roos for Huddersfield Town loanee Ben Hamer after he conceded three goals at Fulham on 26 November 2019. He worked on his weaknesses with goalkeeping coach Shay Given and continued to play in the FA Cup. He volunteered money and time to charities during the COVID-19 pandemic in England.

On 16 September 2017, Roos returned to League Two on an emergency loan to Port Vale; the loan was approved as first-choice goalkeeper Rob Lainton was injured and manager Michael Brown did not have confidence in inexperienced duo Sam Hornby and Ryan Boot. He made his debut for the “Valiants” later that same day in a 1–1 draw with Forest Green Rovers at Vale Park, which proved to be Brown’s last game as manager. Seven days later he spilled a routine cross to allow Yeovil Town the equalising goal in a 1–1 draw, but was backed by caretaker-manager David Kelly, who said “he is a good presence for us and he will get over it”. He played nine matches before he was forced to return to Derby after picking up an injury in October.

On 8 December 2017, he then joined League One side Plymouth Argyle on an emergency loan after injuries to Luke McCormick, Kyle Letheren and Robbert te Loeke. In doing so he was forced to cancel a planned trip back to the Netherlands to spend Christmas with his family. Roos played in four games for Argyle, going unbeaten and keeping one clean sheet before leaving Home Park on 28 December; Argyle signed Remi Matthews on loan from Norwich the day later.

On 1 February 2016, Roos joined League Two side AFC Wimbledon on loan for the rest of the 2015–16 season. He made his debut for the “Dons” 12 days later in a 4–1 victory over Luton Town at Kingsmeadow. He quickly established himself in Neal Ardley’s first-team, and proved highly popular with supporters as he kept eight clean sheets in 20 appearances for the club. He helped the club achieve promotion to League One for the first time since their reformation, as he kept another clean sheet in a 2–0 victory over Plymouth Argyle in the League Two play-off final at Wembley Stadium, making an “excellent diving save” from a Graham Carey free-kick when the game was still goalless.

On 24 August 2016, Roos signed with League One side Bristol Rovers on a season-long loan. He made his debut for the “Pirates” six days later in a 3–2 EFL Trophy defeat by Reading U23 at the Memorial Stadium. He made his league debut for the “Gas” on 10 September, in a 2–2 draw with Rochdale. However he was dropped in December as manager Darrell Clarke decided to give Will Puddy a run in goal. He returned to Derby on 10 January 2017, having made 22 appearances in all competitions for Bristol Rovers.

On 15 July 2015, Roos joined Championship rivals Rotherham United on a season-long loan deal, along with team-mate Farrend Rawson. However he got off to a poor start with a 4–1 defeat to Milton Keynes Dons at the New York Stadium on 8 August, which led to heavy criticism from “Millers” fans on social media. Roos returned to Derby just six weeks into the loan period after conceding ten goals in his four appearances, leaving Rotherham bottom of the table. Manager Steve Evans stated that “at this minute in time his inconsistency is not something we could afford”, and signed free agent Lee Camp to replace him.

 — Derby County first team coach Paul Simpson on Roos.

On 23 January 2014, Roos moved to Championship side Derby County after impressing during a trial. He signed a ​2  ⁄2 -year contract for an undisclosed fee, believed to be around £30,000 plus add-ons, including the requirement that Derby play a pre-season friendly with Nuneaton in the summer. He was installed as understudy to Lee Grant after the departure of goalkeeper Adam Legzdins, and started the 2014–15 season on the bench. He made his first-team debut for the “Rams” in a League Cup match against Reading at Pride Park Stadium on 23 September 2014. He made several saves and kept a clean sheet as Derby won 2–0. He played in Derby’s next League Cup match, a 5–2 win over Fulham on 28 October, and also played three FA Cup matches as Derby recorded 1–0 and 2–0 wins over Southport and Chesterfield before Reading gained revenge by eliminating Derby from the FA Cup at the Fifth Round.

Having received offers from the Netherlands and England, Roos went on to join Conference Premier side Nuneaton Town on 8 November 2013, signing a contract until the end of the 2013–14 season. His “Boro” debut was delayed by almost two weeks as the club waited to receive international clearance. There were initial rumours and controversy over his debut, with led chief executive Ian Neale to deny that any pressure was placed on manager Kevin Wilkin to play Roos. His debut came on 23 November, in a 1–0 victory over Chester at Liberty Way, where he earned praise for a “fine” late save from a close-range Jason Jarrett strike. He played a total of nine league matches, keeping five clean sheets, and also played four cup matches, keeping three clean sheets, not conceding a goal in his first six appearances. His form drew the attention of Premier League club Liverpool, with whom he spent a week’s trial in early January 2014, as well as Everton, Sunderland and Cardiff City. He also spent time training with Ipswich Town and Bolton Wanderers and was scouted by Crystal Palace, Queens Park Rangers and Bournemouth.

Kelle Willem Roos (born 31 May 1992) is a Dutch professional footballer who plays as a goalkeeper for Championship club Derby County.

He spent his youth with a wide variety of Dutch clubs, including: VV Toxandria, VV Trekvogels, Juliana ’31, Quick 1888, PSV Eindhoven, Willem II, N.E.C., and Nuenen. He earned a contract at English Conference Premier side Nuneaton Town in November 2013, and gained a lot of attention from clubs throughout England with his performances during a run of clean sheets. He was sold on to Derby County for £30,000 in January 2014, and made his first-team debut for the club nine months later. He joined Rotherham United on loan in July 2015, but struggled with inconsistency and had his loan spell cancelled after just six weeks. He joined AFC Wimbledon on loan in February 2016, and enjoyed a much more successful spell, keeping goal as the club won promotion with a 2–0 victory in the 2016 League Two play-off final. He spent the first half of the 2016–17 season on loan at Bristol Rovers, and joined Port Vale and then Plymouth Argyle on emergency loans in the first half of the 2017–18 season. He broke into the Derby first-team in January 2019.

Roos began his career at amateur Dutch clubs VV Toxandria, VV Trekvogels, Juliana ’31 and Quick 1888, before moving to PSV Eindhoven in 2005. After six years at the PSV Academy he moved to Willem II, where injuries hampered his progress, before moving to N.E.C. in 2012. At the start of the 2013–14 season, Roos had a trial at English Championship club Birmingham City and then at League One side Bristol City. He competed with Elliot Parish for the chance to win a permanent contract, with manager Sean O’Driscoll then signing Parish permanently. O’Driscoll also wanted to sign Roos and loan him out, but financial restraints left him only able to sign one of the pair. Having failed to earn a contract and unable to join a league side as the transfer window had closed, Roos returned to the Netherlands and signed for amateur Hoofdklasse side Nuenen.