Mata has played his way back into manager Jose Mourinho's plans in recent weeks.
Lampard said: "Juan has been a vital player ever since he came to the club and he showed that against Tottenham. He's been first class. Every player here understands that when you look around the dressing room with the squad that we have, there is a lot of competition, especially in the attacking midfield area.
"People have to be at the top of their game and if they are not playing - we have all been there - you have to stay and train well, keep the right attitude. Juan has done that."
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