Manchester United manager Louis van Gaal hailed his new-look strikeforce on Saturday after the Premier League giants scored a morale-boosting 3-1 friendly victory over Barcelona.

Wayne Rooney scored the opening goal before second half strikes from Jesse Lingard and Adnan Januzaj settled an entertaining game in front of 68,416 at the Levi's Stadium, home of the NFL's San Francisco 49ers.

A satisfied van Gaal hailed the burgeoning attacking partnership between Rooney and new signing Memphis Depay following the win.

"They have always a threat and that's very important," van Gaal said. "I think Barcelona could not cope with the threat of Depay and Rooney."

Van Gaal also saluted the scoring cameo of young Belgian international Januzaj.

"I have to say that Adnan was also a big threat to them in the 30 minutes that he played, so that's good to see," he said.

However van Gaal cautioned that United's form on their US tour, which wraps up on Wednesday against Paris Saint-Germain in Chicago, would count for nothing if they failed to beat Tottenham in their opening Premier League game next month.

He pointed out that United had also beaten the then reigning European champions -- Real Madrid -- in pre-season last year, before losing to Swansea in their opening league match.

"We won last year (against Real Madrid) and it doesn't mean anything because we lost the first match of the Premier League season against Swansea," van Gaal said.

"That's why I have said the whole tour, the most important game is not against Barcelona, Paris Saint Germain or Club America it is against Tottenham Hotspur."

Barcelona, missing superstars Lionel Messi and Neymar after their exertions in the Copa America, were left ruing a series of chances which hit the woodwork. Their lone goal came in the 90th minute via Rafinha.

Barcelona could have been 2-0 up within the opening minutes, Luis Suarez crashing a free-kick off the post after only four minutes with United goalkeeper David De Gea beaten.

Moments later De Gea was pressed into action when Sergi Roberto burst through only to shoot directly at the United keeper with the goal at his mercy.

United made Barcelona pay for their early profligacy in the seventh minute when Rooney headed the Red Devils into the lead, punishing slack marking to power Ashley Young's corner into the net past Marc Andre Ter Stegen.

The goal boosted United and they grew in confidence, with Depay's movement and interplay with Rooney again catching the eye for United.

Young meanwhile should have doubled United's lead in the 28th minute.

The left winger advanced purposefully into Barcelona territory and as defenders backed off was presented with a clear shot at goal.

The midfielder's shot was too close to Ter Stegen however and the Barcelona keeper was able to parry a reflex save to safety.

At the other end Barcelona continued to chase an equaliser and Suarez was left cursing the woodwork once more on 43 minutes when his shot from the edge of the area hit the opposite upright.

Barcelona were unlucky not to claim an equaliser just after the hour mark, when Suarez looked to have been fouled by Daley Blind in the penalty area.

In the same move Suarez then finished low past De Gea only to be ruled offside.

United rang the changes on 63 minutes, removing their entire starting XI as van Gaal indicated he would do before the match.

The change yielded instant results, with Tyler Blackett crossing low from the left for Lingard to tuck away the finish for 2-0.

United might even have extended their lead moments later when Januzaj latched onto a long ball and shot just wide.

Barcelona kept probing for a goal, only for the woodwork to deny them once more in the 72nd minute when Munir El Haddadi's header struck the post.