Just as David Moyes finally had something to cheer, the Manchester United boss has been rocked by the news that Robin van Persie will be out for up to six weeks with a knee injury.

Moyes’ first season in charge had continued to clatter from one disaster to the next, with an insipid 3-0 home defeat to United’s biggest rivals Liverpool on Sunday putting Sir Alex Ferguson’s successor under fresh pressure. But with his reign at its lowest ebb, United finally showed some signs of life on Wednesday when they overturned a 2-0 deficit against Olympiakos, with a 3-0 win at Old Trafford sending them through to the Champions League quarterfinals.

The positive signs did not last for long. Already dealing with the news that United’s prize for their victory is a meeting with Champions League holders and favorites Bayern Munich, it was then revealed that the man whose hat-trick kept United alive, and potentially Moyes, in a job, could be missing for the rest of the season.

United will be without Van Persie then when they travel to London to take on West Ham on Saturday evening. Speaking before the extent of Van Persie’s injury was known, the United boss admitted that the win over Olympiakos has lifted a significant burden off his shoulders.

“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," he said, according to Reuters.

"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."

Nemanja Vidic missed the midweek match with a groin injury, and will definitely be absent against West Ham having been sent off against Liverpool last week. Antonio Valencia is a doubt after suffering a blow to his eye against Olympiakos, but Moyes could make several changes at Upton Park with Tuesday’s encounter against local rivals Manchester City in mind. Juan Mata is likely to return to the side having been ineligible for the Champions League.

Languishing in seventh place in the Premier League, Manchester United’s season is effectively all-but over domestically. The same is far from true for their opponents. A run of four straight wins looked to have lifted West Ham out of any danger, but back-to-back defeats now sees them only six points above the drop zone and with third from bottom Sunderland having played two games less. Manager Sam Allardyce has revealed that he is focusing on the side’s home form to lead them to safety.

“Our efforts at home are what I've been focusing on with the players, the fact that we've won the last three home games on the trot and we have to try and maintain that if we can to overcome this little blip,” he said, according to West Ham’s official website.

"We've had two away games in which we haven't picked anything up in, so the focus is to maintain our home results and try our best to pull off a big result. Try and win, but if we don't then make sure we come away with at least a point.”

Allardyce has no fresh injury worries, with only Joey O’Brien and on-loan Italian striker Marco Borriello unavailable. 

Where to watch: The Barclays Premier League encounter will kick off at 1.30 p.m. ET. Coverage will be provided by NBCSN, with a live stream available via NBC Sports Live Extra.