Juan Mata scored the only goal of the game at Old Trafford as time ran out after David De Gea made a number of crucial saves to keep the scores goalless.
And despite an upturn in form in recent weeks, Van Gaal insists the problems from before are still there for the Red Devils.
“Of course, four victories in a row is fantastic but we have a very small selection [of players] at this moment,” said the Dutchman.
“We have the same problems as three weeks ago. We have some players coming back but still I have to solve a problem in my line up.
“When players are coming back, they are not fit enough. I think we are doing great the last four matches but still we have to go step by step.”