Hiddink was asked about United's troubles in a campaign which has seen Van Gaal's position come under sharp focus amid fan frustration over tactics and results, with Jose Mourinho apparently waiting in the wings to succeed him.
Van Gaal has bristled at suggestions he could lose his job, with United fifth, 10 points behind leaders Leicester.
And a win at Stamford Bridge on Sunday could reignite the Red Devils' hopes of succeeding Chelsea as Premier League champions.
"He has to perform, this is normal, as we have to perform here as well," Hiddink said.
"There is pressure
There was and still is big ambition
They have to cope with that.
"The pressure is on
It's on everyone
But if you have the possibility to go into first position, then pressure is on."
Hiddink replaced Mourinho at Chelsea in December and declined the opportunity to fully appraise United's season, suggesting the Blues had enough concerns of their own.
The London side remain in the bottom half of the table entering this weekend's fixtures.
"I am not going into the 'problems' from other clubs as well," Hiddink added.
"We can talk all morning about Manchester United, but I'm wearing a different colour.
"Every club during a season has some periods where they might worry a bit.
"We had that as well
I don't like to go in too much in other people's kitchen."
Chelsea are 16 points adrift of Arsenal in fourth, the final Champions League qualifying place.
Hiddink has overseen two wins and five draws in his unbeaten seven-match Premier League run and, with the top four seemingly out of Chelsea's reach, he hinted the priorities are now the FA Cup and Champions League, where Manchester City and Paris St Germain, respectively, are the opposition.
"To get fourth position you need to win, win, win
That's very, very difficult," he added.
"The other two legs are FA Cup and Champions League
That's where we're aiming."
Chelsea's troubles and likely lack of European football - unless they win the Champions League, like in 2012 - could lead to a high player turnover this summer.
Ramires has already left for the Chinese Super League and it was reported that Oscar was the subject of a £57million bid from Jiangsu Suning.
"Ramires, we can say he had a great offer
Oscar is just for me, speculation, and we don't go into that," Hiddink added.
"We don't know anything about this."
China could also be a summer destination for captain John Terry.
Chelsea maintain Terry could yet be offered a new contract after his public declaration he would be leaving at the end of the season.
The emergence of China and the financial clout is "a concern" for Hiddink, who has had discussions with the football hierarchy there.
He said: "It's a concern
It shows China is very serious to get into international football and the recognition of the football world.
"It's relatively easy to bring in big money and to buy star players, but it's equally important for China, for the development of Chinese football, to bring in very high qualified people to educate.
"In my opinion, you cannot just buy with a lot of money big star players and think you have grounded Chinese football.
"I know from their side, even from high-ranked government people, they are having a very serious approach towards Chinese football.
"That's a step in the direction that will not be profitable tomorrow, but it will be in future years."
Source : PA
Source: PA