United fans booed loudly when Van Gaal substituted Anthony Martial for Marouane Fellaini in the 66th minute of Tuesday's 1-0 win over CSKA Moscow.
The supporters were clearly unhappy about the often sterile style of play as they also sang: "We're Man Utd, we want to attack."
Van Gaal admits supporters are entitled to their opinion, but believes negativity is seeping through to the players.
"The supporters have to support the players otherwise they make it very difficult for the players to play at Old Trafford," the United manager said.
"I can only advise the fans to criticise the manager and not the players
It's very difficult to play for Manchester United with a lot of pressure and you can taste that in Old Trafford because of all the yelling, then it's not good for my players," Van Gaal added.
"Then it's better to whistle the manager, I can cope with it because I have had a lot of experience in my life as a manager."
Source : PA
Source: PA