In the following two seasons, Mata was an everpresent offensive figure for Valencia, scoring 17 goals in 68 games combined, with the club achieving back-to-back third-league places. On 10 April 2011, he netted two in a 5–0 home win against Valencian neighbours Villarreal CF. On 9 May, England-based Spanish journalist Guillem Balagué reported interest from several Premier League clubs.
On 21 August 2011, Valencia announced they had agreed a £23.5 million fee for the transfer of Mata to Premier League club Chelsea, subject to a medical. On 24 August 2011, Mata signed a five-year deal with Chelsea. He revealed that compatriot Fernando Torres helped persuade him to move to Stamford Bridge, saying "Fernando got me excited about the thought of coming here. He said it would be good for me here, and that me and him together could be good. I also talked to my family and friends about it as well."
On 26 August 2011, Yossi Benayoun offered Mata his number 10 shirt. The Israeli captain said, "I decided to give Mata the number 10 – his favourite. For me, it's just a number, not my lucky 15." (Florent Malouda wore number 15 for Chelsea at the time.) Mata wore number 10 for Valencia and for Spain's under-21s in the summer, and expressed his gratitude to Benayoun. "It is a very important number to me so I'm pleased to be wearing it. I want to thank Yossi," he said. Mata followed on from Chelsea players Joe Cole, Mark Hughes, Ian Hutchinson, and Terry Venables in adopting the number 10 shirt.
Mata made his debut for Chelsea against Norwich City on 27 August 2011, where he came on as a 68th-minute substitute for Florent Malouda and scored the club's third goal in the 11th minute of stoppage time. Chelsea went on to win the match 3–1. He started his first game for his new club in their next fixture on 10 September away to Sunderland, helping Chelsea secure a 1–2 win. On 14 September, Mata made his UEFA Champions League debut for Chelsea and netted in stoppage time to help his side defeat Bayer Leverkusen 2–0.
On 29 October 2011, Mata assisted Chelsea's 6,000th league goal with an in-swinging cross which was headed in by Frank Lampard. In the same game, Mata scored with a spectacular 25-yard strike in a 3–5 home defeat to London rivals Arsenal. On 26 November, Mata set up the first two goals which were scored by John Terry and Daniel Sturridge and also scored the third goal himself in a 3–0 win against Wolverhampton Wanderers. He was awarded the Man of the Match in a 0–3 away win at St James' Park for his performance against Newcastle United.
On 6 December 2011, during Chelsea's decisive Champions League match against former club Valencia CF, Mata created two assists for both of Didier Drogba goals. The game ended in a 3–0 win, which resulted in Chelsea qualifying first from their group. On Boxing Day, Mata scored his side's only goal in a 1–1 draw at Stamford Bridge against Fulham. Mata scored Chelsea's first goal of their FA Cup campaign in a 4–0 win over Portsmouth at Stamford Bridge.
In 2011, the following summer, he went back to the under-21 side, playing the role of senior squad member and helped Spain win the 2011 UEFA European Under-21 Championship in Denmark. Mata himself was included in UEFA's Team of the Tournament and named as the tournament's Golden Player.