Romeu made his first La Liga appearance on 15 May 2011, playing the last ten minutes of a 0–0 home draw against Deportivo de La Coruña in the place of another club youth graduate, Jonathan dos Santos.
Reports began to link Romeu with a move to Chelsea on 22 July 2011. The following day he agreed terms for his transfer, pending a medical or personal terms as he was away on international duty at the 2011 FIFA U-20 World Cup. On 4 August the English club announced the completion of the transfer, with the player signing a four-year contract in a deal worth €5 million – the deal also gave Barcelona first refusal via a buy back option should Chelsea deem him surplus to requirements, seeing Barcelona pay €10 million in 2012 or €15 million in 2013 to bring him back to the Camp Nou; he was given the number 6 shirt for the new campaign.
Romeu made his official debut for Chelsea on 10 September 2011, being brought on as a substitute in the 79th minute of the 2–1 away win against Sunderland. He received his first start on the 21st, in a 0–0 draw (penalty shootout win) against Fulham for the season's Football League Cup.
Romeu's first UEFA Champions League appearance came on 19 October 2011, playing the full 90 minutes in a 5–0 group stage home win against K.R.C. Genk. His first start in the Premier League was against Wolverhampton Wanderers on 26 November, putting in a solid performance in a 3–0 home win.
In December 2011, Chelsea issued a statement regarding the deal surrounding Romeu's buy back clause with Barcelona. The statement highlighted that the Blaugrana did not have an automatic buy back clause as first suggested, only having first refusal on any deal if the Blues wanted to sell him in the future.