The Red Devils host Everton at Old Trafford and van Gaal suggests he may use the Spanish playmaker to step into Wayne Rooney's position while the captain serves a three-match suspension for last weekend's red card in the 2-1 win over West Ham United.
"I will miss Wayne because he is the captain of the team but Mata can also play very well," van Gaal said in the pre-match press conference.
"I have to choose. Juan Mata has played well in his (Rooney's) position."
"We have eight injuries and one red card, so nine players are not available," he added.
"It's difficult to explain. You can be fit but not match fit - that's a big difference.
Seventh-placed United will be looking to string two wins together for the first time this season when the struggling Toffees come to town with Roberto Martinez's side in 15th position with just one league win.
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