Mata made the difference as the Blues kept themselves on course for Champions League football next season - their prime remaining objective of the campaign - with a 1-0 win at Manchester United.
The 25-year-old told Chelsea TV: "It wasn't our best game."
Speculation has continued that Jose Mourinho will be the man at the helm for the 2013-14 season after a Spanish TV show aired a picture of him shopping in Ikea in Madrid. Punto Pelota claimed the Real Madrid coach spent 200 euros on packing boxes and masking tape with the implication being it could be in preparation for a house move.
Should Mourinho return, he is sure to want to build his team around men like Mata. The Spaniard's decisive strike three minutes from time, although recorded as a Phil Jones own goal after a deflection off the defender, lifted the Londoners back into third in the Barclays Premier League with a priceless win.
With a strong goal difference in their favour, they can now effectively secure a top-four place with victory over rivals Tottenham at Stamford Bridge on Wednesday. That would then put them in good heart as they step up preparations for the Europa League final against Benfica later this month.
But with Chelsea having already played a club-record 65 games - after campaigns in the Champions League, Europa League, Club World Cup and extended domestic cup runs - Mata concedes the players are feeling the effects.
Mata added: "I felt in the first half, and sometimes second half, we played a bit slow, maybe because of the amount of games that we have already played during the season.
"I think it is normal. When you arrive at the end of the season your legs are a bit tired, mentally as well - but you have to be ready because it is the most important part of the season.
"I think we fight until the end."
- Sports & Recreation
- Jose Mourinho