The Argentina international suffered the problem in the second half of Sunday’s 1-0 defeat by Manchester City and joins Radamel Falcao, Jonny Evans and Phil Jones on the sidelines.
“Marcos dislocated his shoulder but did it in a positive way,” said Van Gaal on Friday.
“We don’t need to operate on him. It’s a relief but maybe, with this injury, it is very difficult to prevent it from happening again. That is the problem.
“He saw a specialist yesterday evening who said the conservative way to heal it is best. For him, it was positive. Always with these injuries, I cannot change the human body. God made it – not me. Conservatively, [he’ll have] less than six weeks out.”
Asked about Falcao’s fitness, Van Gaal replied: “The problem for Falcao is because of balance in the game.
“I had to change, he came in and assisted a goal. Then in training he was injured.
“I cannot judge now because he is injured and I changed him not because of his performance but because of the balance of the team, which is much more important than the individual.”
Michael Carrick, Paddy McNair, Tyler Blackett and Tom Thorpe are among Van Gaal’s limited choices to replace Rojo at centre-half for the visit of the Eagles on Saturday.
“When I want to select the same line-up, I cannot,” added van Gaal.
“It’s the same every week. It is like it is and I don’t want to complain or cry because I have great belief in the players, myself and my staff. Also the supporters who are supporting us despite the results.
“Smalling is suspended. Evans and Jones are still injured. So I have to solve a problem but I don’t want to say who is playing yet.”
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