The midfielder was hooked during defeat at Everton.
Mata said: "We need to get the club's morale back, which has been knocked.
"We know we enter the decisive moments of the FA Cup and the Champions League.
"The problem with our game at the moment is not just down to the amount of goals but also the quality of our play.
"The way we are playing at the moment is inferior to what we were producing in previous months."