Van Gaal has been tasked with bringing the glory years back to United following last year’s seventh-place finish under David Moyes.
Ancelotti did the double in his first season at Chelsea but, even though he rates Van Gaal highly, the Italian thinks it will be difficult for the Dutchman to lift the trophy at the first time of asking.
"They signed really good players this year but to build a team is not so easy," Ancelotti told a press conference on the eve of Real’s friendly against United in Ann Arbor.
"The Premier League is a very competitive league and it will not be easy to win it the first year because there are a lot of teams who are really competitive and they want to win."
Ancelotti dodged a question on whether he was approached by United following Moyes’ sacking.
"These are rumours that usually happen to the manager," he said.
The former Chelsea manager thinks the United hierarchy chose wisely when they appointed Van Gaal, though.