Louis van Gaal wants to add to his Manchester United squad in the upcoming transfer window but says January is a "more difficult" market than the close-season.

United sit third in the Premier League table ahead of Saturday's home game against West Ham but, despite being just one point adrift of leaders Manchester City, Van Gaal has faced criticism for his side's functional and often underwhelming recent performances.

The Old Trafford club have scored just seven goals in their last nine matches in all competitions and are suffering with a raft of injury problems, leading to speculation that Van Gaal may attempt to bring in more firepower when the transfer window reopens next month.

However, speaking at his pre-West Ham media conference on Friday, Van Gaal sounded sceptical on his chances of strengthening his squad before the end of the season.

"They are always contact injuries and you cannot afford that. We have a lot of contact injuries and also long-lasting injuries so that's our problem," the Dutchman explained.

"So we are looking [at players for January] but as you know it is not so easy to buy players and in January it is more difficult. We are looking but it is not for sure."

Van Gaal also revealed that captain Wayne Rooney is out of Saturday's match and may miss next week's crucial Champions League trip to Wolfsburg due to a "nasty" ankle injury, while defender Marcos Rojo is facing a spell on the sidelines after dislocating his shoulder in training.