Manchester United’s positive week took a turn for the worse this evening when striker Robin van Persie was ruled out for “around four to six weeks” with a sprained knee.

The Premier League champions announced the bad news on their official Twitter page, with the Holland international cut down just two days after firing the club into the quarter-finals of the Champions League.

Van Persie’s hat-trick against Olympiakos had been viewed as a possible turning point in United’s poor season, but he was later carried off on a stretcher with Friday’s news meaning he may not play again until May.

He is almost certain to miss the Champions League last-eight tie with Bayern Munich, which he single-handedly set up.

A tweet from United read: “Following further investigation, Robin van Persie has a sprained knee, which will keep him out for around four to six weeks.”

Manager David Moyes had earlier suggested that there could be bad news regarding the former Arsenal man.

“We’ve got a lot of games coming up and we need all our players fit and available,” he said.

Van Persie's absence is likely to mean an opportuntiy for Danny Welbeck to impress up front where he has not been able to hold down a regular place. The pacy young England striker, however, kept the Olympiakos defence busy on Wednesday.

That result eased the pressure on Moyes considerably; so much so, in fact, that the manager reckons he can look supporters in the face again after their season went from bad to worse with Sunday's 3-0 defeat to Liverpool at Old Traffford.

“In some of the games I have felt ashamed because we haven’t played well enough and I have not got a team out that has got a good enough result,” Moyes said.

“I think the big thing on Wednesday was that I could look at the supporters in the face after the game. We had given them something to shout about. There was nothing to be ashamed of about our performance.”

With a top-four finish in the Premier League now looking almost impossible for United, who are 12 points behind fourth-placed Manchester City, winning the Champions League is their only realistic hope of qualifying for it next season.

United are the least favoured of the eight teams left in the draw but Moyes said he was just glad to be still in the mix.

“In a lot of ways it didn’t matter who I drew but I’ve probably drawn out the favourites,” said Moyes, whose side will be at home in the first leg next month.

“They are the holders as well, so it’s a tough draw but one I’m looking forward to. I’ve got us to the quarter-final and now the aim is to get to the semi-final.

“If you’re old-fashioned you’ll probably say you want to be at home in the second leg and that would be the preference but we’ll just have to take it.”

United will look to build on their win over Olympiakos at West Ham United on Saturday when they will be without Van Persie who injured his knee on Wednesday.

“There is no guarantee that one good result will lead to another,” Moyes said. “But you would rather have a good one than a bad one and we got a really good one in midweek.

After the trip to West Ham United have a home derby against City on Tuesday, and must now plan for that without Van Persie.