The Spain international was left on the bench for the trip to Moscow after the 26-year-old started all nine of their Premier League games and their previous two Champions League group-stage games.
The Red Devils were forced to come from behind against CSKA Moscow after 19-year-old Anthony Martial scored a second-half equaliser to secure a point in Russia.
Speaking to United’s website, Van Gaal explained that Mata needed a rest ahead of the visit of Premier League leaders Manchester City at Etihad Stadium on Sunday afternoon.
“I have to rotate,” Van Gaal told United’s website. “Juan Mata has played almost every game. He needs a bit of a rest. We are always here to beat the opponent.
“They have a speedy attack and I expect them to play on the break.”
Mata has scored three goals and has made three assists in nine Premier League games this term.
The World Cup winner has also netted one goal and made game in two Champions League outings.
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