Wayne Rooney has suffered a knee injury that may rule him out for at least six weeks in a major blow to Manchester United’s hopes of success this season. The club’s top scorer, who recently registered seven times in as many games, is expected to return no sooner than the start of April.

It would mean Rooney will be unavailable for a minimum of six matches, including two rounds of the Europa League, should United progress, and Monday’s trip to Shrewsbury Town in the FA Cup fifth round.

Rooney, 30, will also be absent for England’s friendlies against Germany and Holland at the end of March, as the manager, Roy Hodgson, begins preparations for Euro 2016.

The Europa League and FA Cup represent United’s most realistic chance of claiming silverware this season given a faltering league campaign that has left Louis van Gaal’s side in fifth position, six points behind Manchester City in the final Champions League place with 12 games left.

With that in mind, United’s manager and Rooney have each identified winning the Europa League as potentially United’s best chance of qualifying for the Champions League.

When Rooney suffered the injury is unclear, though Van Gaal indicated the captain may have played on with it during Saturday’s 2-1 defeat at Sunderland. It forced Rooney’s removal from the 18-man squad for Thursday’s Europa League last-32 first leg at FC Midjytlland.

While Van Gaal would not put a time on how long Rooney will be missing, it is thought to be a minimum of six weeks. Speaking at MCH Arena in Herning, home of FC Midjytlland, Van Gaal said: “It’s difficult to say but he was injured in the game versus Sunderland and has a knee problem. We have to wait and see how long that takes. That’s also typical Wayne – he’s a guy who wants to go until the end and if he is feeling pain, he doesn’t want to go away, and certainly when you are drawing a match because he wants to win. That’s a fantastic attitude but also it’s bad for his body. You do not know when it happened, so that’s the difficulty.”

Rooney had recently been in fine form with his seven-goal run, between the 2-1 victory over Swansea City on 2 January and the 3-0 victory against Stoke City on 2 February, although he failed to find the net in United’s last two games. This season he has scored 14 times in 32 United appearances, plus three goals in four matches for England which made him his country’s record scorer with 51.

Anthony Martial is United’s next highest scorer with 10 in 31. The 20-year-old Frenchman, who was signed for an initial £36m in the summer, started the season as the No9 before operating in a wide left berth. Apart from him Van Gaal has no other recognised striker at the club. Memphis Depay has managed five goals in a disappointing debut campaign.

Van Gaal said: “We have in Martial another striker, he has played several matches in this role and wants to play in it. I knew in advance that Rooney would play a lot of matches, then I have called Will Keane back [from loan] and he has done very well in the second team. Sometimes you have to show it at the higher level and maybe he’ll get the chance to show it – you never know.”

Van Gaal continues to be scrutinised by Ed Woodward, the executive vice-chairman, regarding his position. So Rooney’s absence could also affect the Dutchman’s job security. He is the 10th major player to become unavailable due to injury.

Last season under Van Gaal there were a high number of injuries and he may have to explain to the club hierarchy why he has a relatively small squad of 23 first-choice players. He said: “Our policy was not to have a big squad as we wanted to try youngsters in our team. Normally we don’t have 13 injured players. It’s ridiculous when you analyse it – they’re all contact injuries. With that policy, normally once or twice you have to back up the team with youngsters but injuries have happened every week this season.”