Memphis Depay, Antonio Valencia and James Wilson are injured ahead of Manchester United's trip to Crystal Palace in the Premier League on Saturday.
"Willo was injured but also Memphis Depay was injured," Van Gaal told MUTV on Friday.
"It was a very bad game afterwards because we lost on penalties and also we lost two players. We have to wait and see [how long they are unavailable for] because it was only two days ago."
Depay, 21, has made eight Premier League appearances for United since signing from PSV Eindhoven in the summer, scoring one goal.
Information from the Press Association was used in this report.