Third-placed United will arrive at Stamford Bridge on Saturday on a run of six league victories but last weekend's 4-2 derby demolition of champions Manchester City came at a cost.
Midfielder Michael Carrick has a calf problem, Daley Blind sustained an ankle injury in a hefty challenge from City captain Vincent Kompany and defenders Phil Jones and Marcus Rojo also picked up knocks.
"It is the worst scenario," Van Gaal told a news conference on Friday.
"Marcos and Phil can both play left central defender and Jonny Evans is also suspended so I do not have many choices.
"Daley Blind and Michael Carrick could play as a holding midfield player and I do not have another option. It is a puzzle."
Van Gaal, who struggled with a lengthy injury list at the start of the season, may deploy captain Wayne Rooney in midfield alongside former Chelsea midfielder Juan Mata.
Jose Mourinho's Chelsea have a seven-point lead and a game in hand at the top but the west Londoners have looked less than convincing in recent weeks and laboured to a 1-0 victory over Queens Park Rangers last weekend.
United would move above second-placed Arsenal, who play Reading in an FA Cup semi-final on Saturday, and within five points of leaders Chelsea with victory at Stamford Bridge.
Despite their recent hot streak, however, Van Gaal believes winning the title is out of United's reach.
"It is still possible," Van Gaal said. "But it is not logical. It is more 'when' and 'if'. I don't believe in 'when' and 'if'. I believe in facts.
"We can lay pressure on Chelsea's shoulders and then you never know. I think Chelsea is satisfied with a draw. Of course, they want to beat Manchester United and we want to beat them."

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