Robin van Persie may not play for Manchester United again this season due to a sprained knee, with the striker concerned his World Cup hopes could be ruined if he attempts to return too early.
Van Persie will miss both legs of United's Champions League quarter-final against Bayern Munich next month, and is scheduled to return to fitness just days before the final Premier League fixture of the season, on 11 May.
Holland begin their World Cup campaign on 13 June against Spain, but Van Persie will need to have proved his fitness by 2 June to be included in Louis van Gaal's squad for Brazil.
While the club stated the Dutchman would miss "four to six weeks", it is understood that Van Persie, who had the injury rescanned in Holland on Friday, has been informed of the need for caution.
The striker is to undergo treatment on the knee and even if Van Persie waits the full six weeks, which would take him into early May, he will have to be very careful returning then so as not to compromise playing at Brazil 2014.
Van Persie suffered the injury towards the end of the 3-0 Champions League last-16 win over Olympiakos on Wednesday at Old Trafford, in which he scored all of the goals.
The problem occurred when he was hit from behind in the last minute by the centre-back Kostas Manolas, and collapsed on the turf clutching the back of his left knee .
He had to be taken off on a stretcher but at the time Moyes downplayed the problem, describing it as "a knock in the back of his knee" that "hopefully is not too bad".
Given that Van Persie's three goals took him to 14 in 23 United outings, making him the club's top scorer this season, his loss is a big blow to Moyes. After a virtually injury-free season last year, Van Persie has again been troubled this season as he was for most of his Arsenal career.
Van Persie suffered a muscle injury during United's summer tour of Japan, issues with his toes and groin once the season started, and a problem in mid-December that put him on the sidelines for six weeks.
Moyes also faces a crisis in central defence due to Nemanja Vidic being suspended for two games, with the captain also having a groin injury. Chris Smalling and Jonny Evans are injured too, with hamstring and calf problems respectively. This rules each out of this evening's match at West Ham, almost certainly Tuesday's derby against Manchester City, and both could even be unavailable for the first meeting with Bayern on 1 April, according to Moyes.
"I couldn't give you an answer just now," he said when asked if Evans and Smalling could face the Germans. "The defence is a problem because we are missing Chris Smalling and Jonny Evans but we are desperately trying to get them back and available – we have other important games that we need them ready for."
United's victory over Olympiakos, in which they overturned a 2-0 first-leg deficit, has restored Moyes' self-belief, with the manager having felt "ashamed" after some previous displays.
"The big thing was that I could look the supporters in the face after the game on Wednesday night because I thought that we had given them something to shout about and we had nothing to be ashamed of in our performances," the Scot said. "In some of the other games I've felt ashamed because the team haven't played well enough and I haven't got a team out that has got a good enough result."