MANCHESTER (REUTERS) - Manchester United manager Jose Mourinho fears the injuries suffered by striker Zlatan Ibrahimovic and defender Marcos Rojo are serious, and could result in lengthy absences from the team.

Both were substituted with knee injuries during United's Europa League extra-time quarter-final win over Anderlecht on Thursday.

"I want to wait," Mourinho said when asked about the injury status of the duo.

"But my feeling is not good for both. I want to wait and to be optimistic but I am not," Mourinho told reporters.

Ibrahimovic is United's top scorer this season, with 17 goals in the Premier League, while Rojo has established himself at the heart of United's defence.

Mourinho is currently without two other central defenders, Chris Smalling and Phil Jones, and he urged them to make a special effort to return quickly from injury.

"We have Daley Blind who has experience in the position, and I think it is time for Jones and Smalling to be brave and to risk because for the team you have to do everything, that is the way I think.

"You cannot do miracles, so miracles I don't expect, but you have to do everything to accelerate the process... with a special spirit you can (come back) sooner than expected. So instead of coming mid-May you can come beginning May - this kind of acceleration of the process," he said.

Mourinho also had words of praise for left-back Luke Shaw, who he has publicly criticised in the past.

"Luke Shaw today did something for me new that means a lot for me.

"After 90 minutes, before I knew that I have to change Zlatan, I had Ashley Young ready to come on and (Luke) told me, 'I am going to play this 30 minutes with cramps. No problem if you need to make a change in another position, do it because I am ready for it.'

"These are the kind of things that make me trust players," he said.