Manchester United manager Louis van Gaal has confirmed Marcos Rojo will be out for at least six weeks with the dislocated shoulder he suffered against Manchester City – while Radamel Falcao is also unavailable for Saturday's match with Crystal Palace.

The Argentine international was stretchered off after a collision with Manchester City centre-back Marin Demichelis, and was replaced by youngster Paddy McNair during the 1-0 defeat at the Etihad Stadium.

Rojo tweeted after the game that the injury was nothing serious, while his father claimed he will only miss "two or three" games.

However van Gaal has now claimed that, even though he will not require surgery, the summer signing will be out of action up to six weeks.

"Marcos dislocated his shoulder but did it in positive way. We don't need to operate on him, which is a relief, but maybe with this injury it is very difficult that it shall not happen again. That is the problem," van Gaal said.

"Yesterday evening, he saw a specialist who said the conservative way to heal is best. For him, it was positive. Always with these injuries, I cannot change the human body - god made it, not me. Conservatively, less than six weeks out."

With Jonny Evans and Phil Jones still on the sidelines and Chris Smalling suspended after picking up a red card against City, midfielder Michael Carrick is likely to replace Rojo in the centre of defence alongside youngster Paddy McNair – with Tyler Blackett as the only back-up.

"Smalling is suspended, Evans and Jones are still injured, so I have to solve a problem. I don't want to say who is playing yet. If I say that [what United have done in training] then I give away what I'm trying to do. We've trained today but I may do things differently," the boss said.

Meanwhile, when asked about Falcao's fitness and his adaptation to the Premier League he said: "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."