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.
"He is a good, very experienced manager," Ancelotti added.
"He had a fantastic experience in all the countries he worked in and he will do a fantastic job at Manchester United.
"He had a fantastic World Cup with Holland and is a manager that doesn't have anything to learn."
The build-up to Saturday's game has been dominated by talk of potential incomings and outgoings at the European Champions.
Ancelotti said there was "no news" about Angel Di Maria, the Argentina winger who has been linked with United and Paris St Germain.
"Angel will come back on August 5 as was scheduled before the World Cup," Ancelotti said.
The Italian dodged questions about the futures of goalkeeper Diego Lopez, who could leave if, as expected, Keylor Navas signs for the European champions.
The former Chelsea boss also refused to give any details about Raul, Real's record-appearance maker, who is reported to on the verge of rejoining the club in an executive role.
"I don't have any news about Raul or this topic," Ancelotti said.
Ancelotti did, finally, relay some information to the gathered media - about Cristiano Ronaldo.
The Portuguese will not play against his former employers, Ancelotti said, as he is still lacking fitness following a month off.
"We want to use him in the Super Cup so we won't risk him (on Saturday)," the Real Madrid manager said.
Saturday's game against the Red Devils on the outskirts of Detroit seems somewhat inferior to the clashes Ancelotti has enjoyed against United with Chelsea, Juventus and AC Milan.
But, given that over 109,000 are expected to attend, Ancelotti is looking forward to what he expects to be a tough match.
"It is never a friendly with Manchester United," the 55-year-old said.
"We are happy and honoured to play
It's the last game of our tour.
"We want to go finish well and go back to prepare for the Super Cup.
"We need to play a good game on Saturday because we are playing a really fantastic team."
Source : PA
Source: PA