Manchester United midfielder Michael Carrick has been ruled out for up to three months after undergoing ankle surgery after suffering ligament damage in training.
Carrick went under the knife on Thursday after picking up the injury earlier in the week and has been ruled out for the club's pre-season tour to the United States as well as the start of the 2014/15 season.
Van Gaal had initially confirmed Carrick would be absent for a "long time" after picking up the injury but confirmation of his absence for up to three momths is a major blow to the Dutch coach just days into his tenure at Old Trafford.
"Michael was injured the day before and that's a blow for me because he's experienced," the 62 year old said. "It's very important we have experienced players."
Following the summer departure of Nemanja Vidic nd Rio Ferdinand, Carrick, 32, was among the most experienced players in the United squad and was expected to be crucial in the early days of Van Gaal's reign.
The injury could force Van Gaal to delve into the transfer market once again, having already sign Ander Herrera and Luke Shaw however the former Bayern Munich and Barcelona manager admitted he will assess his squad over the coming weeks before making additional signings.
"Michael was injured the day before and that's a blow for me because he's experienced," said the Dutchman. "It's very important we have experienced players."
"My method is always the same I want to look at the players now," he said. "Of course I know the players but I do not know the players who I have trained and coached.
"I will wait and see after the first three of four weeks and maybe I shall buy other players and the players that we have brought were already on the list. I have my approval because I like them. First I want to see the players play my philosophy."
United travel to Los Angeles for the first game of their American pre-season tour against LA Galaxy before subsequent matches against Roma, Inter Milan and Real Madrid before retuning to English shores to face Valencia.
The club start their season against Swnasea City at home on 16 August.