Manchester United manager Louis van Gaal says he doesn't know if his side can be Champions League contenders.

United did not play in the competition last season after failing to qualify but make their return to the group stage with a clash away at PSV on Tuesday night.

Van Gaal may have overseen more than £250million ($385m) of spending since he took over at United, but he admitted he still does not know whether his squad is capable of being a major force in Europe.

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"We have to show on the pitch if we are able to win matches on the highest level," the United manager said.

"That's also a challenge for us now. Last year we were fourth in the Premier League.

"We qualified ourselves this year and that was our aim so now we have to show if we are able to play at that level and that I don't know, either. I have to wait and see.

"I think the Champions League is a higher podium than the Premier League but maybe it's not like that."

Van Gaal also hinted that Anthony Martial may not start against PSV, despite the Frenchman's goalscoring debut from the bench against Liverpool and the fact that Wayne Rooney is ruled out through injury.

"I've only had one training session after the [Liverpool] match. His impression is good, but you have to give him time," Van Gaal said, speaking at a press conference prior to the Group B match at the Philips Stadion.

"That's why I'm asking the fans. He cannot change his market price."

PSV are managed by former Netherlands international Phillip Cocu, who played under Van Gaal at Barcelona. Cocu guided PSV to a surprise Eredivisie title last season and Van Gaal says he is expecting a tough test in Eindhoven.

"A draw away is always a good result in my opinion," he said.

"But I want to win and the first match, it is very important to win. And it is also like that for PSV, so you have to wait and see."

Van Gaal was linked with the PSV manager's role earlier in his career but insists that will never happen now, as he intends to retire after his role at United.

"It was always dependable on the moment," said the 64-year-old Dutchman.

"Now the moment has passed, so I shall not work at PSV. I will retire after my job in Manchester, so it is not possible."