Mata has been Chelsea's player of the season for two years in a row since his arrival from Valencia in 2011, but has not begun the new campaign as a regular starter and has been told to adapt his game by manager Jose Mourinho.
The Spaniard has responded well to the challenge and made a crucial contribution off the substitutes' bench against Tottenham Hotspur on Saturday as he helped Chelsea battle back to earn a 1-1 draw in the Premier League.
"He played well, linked the play, put in the ball for the goal," said team-mate Lampard.
"He has been a vital player ever since he came to the cub.
"His attitude has been first class. Every player understands there is a lot of competition, especially in midfield.
"If you are not playing, and we have all been there, you have to stay and train well, keep the right attitude. Juan has done that."
While Chelsea dominated the second half at White Hart Lane until Fernando Torres' late sending-off, Lampard was impressed by Spurs in the opening 45 minutes.
Andre Villas-Boas' men have made a strong start to the season after spending heavily in the summer and Lampard believes they must be considered title contenders, although he says the key will be how they deal with the rising pressure of expectation.
"They're a very good team," said Lampard in the Daily Mail. "They are going to be looking at winning the league.
"That's what you have to do when you spend that money. We've lived for the last eight years with that pressure, year in year out, and that's what Tottenham have to deal with if they want to win the league.
"You have to get a level of consistency and it's not easy. It's a difficult league."