United will travel to Villa on Saturday looking to extend their winning streak in the Premier League to seven matches but they will have to do without Argentinian defender Rojo, who is sidelined with a thigh injury.
But in a boost for Van Gaal, Di Maria could be involved off the bench after completing a full week of training.
"It's always dependable on how I have to compare him [Di Maria] with other players because he's not match fit after one week," Van Gaal said on Friday, according to United's website.
"Rafael is not injured," Van Gaal said.
"He was already fit last week, but I have to select and I have to compare.
"When I have more players fit, I have to select. You will have to wait and see, of course. I have to compare.
"Rafael? I can take him in the 18. Rojo? No. We have five injuries and one [Marouane Fellaini] is ill, so they are not coming up. Rojo has a rectus femoris [thigh] injury."
