Mata, who has been demoted to the bench on a number of occasions by Mourinho, came on for the second half ahead of John Terry's equaliser.
"We didn't win but in the second half the speed of our game was brilliant and we upped our level from the first half for sure," Lampard told the club's official website.
"We all changed. Juan Mata came on and made a difference and the other 10 players on the pitch, we upped our level as well from the first half to the second."
Chelsea are currently fourth in the Premier League table after six games.