MANCHESTER -- Ole Gunnar Solskjaer believes a repeat of Manchester United's treble is "impossible" after Liverpool's chances this season were ended by Chelsea.

Liverpool are on course to win the Premier League title and are still in the hunt for the Champions League but were knocked out of the FA Cup at Stamford Bridge on Tuesday.

Solskjaer was part of the United squad which lifted the Treble in 1999 and said the achievement may never be repeated.

"We hope we are going do it again but it's a difficult task," Solskjaer said.

"It's more or less impossible -- there so many good teams around. It has changed domestically. It was either us or Arsenal in '99 and now there's more of us."

United head to Derby on Thursday looking to keep their FA Cup campaign alive. Solskjaer is set to be without Daniel James, who missed the draw with Everton, while Aaron Wan-Bissaka is a doubt because of a back problem.

Marcus Rashford is still absent but Paul Pogba should be ready to resume training with the first team next week.

"Marcus and Paul are still out and I think Aaron might be struggling," Solskjaer said.

"He's got a test today [Wednesday] but he struggled in our last game against Everton with his back so he might be out.

"Paul is still working with the physios outside and he won't be training with the first team until next week. Then let's see how long that will take, he needs time to get his football fitness back."

Solskjaer will be reunited with former teammate Wayne Rooney this week after the former England striker took up a player-coach role at Pride Park.

The 34-year-old has scored four goals in 12 games since returning to England from D.C. United and Solskjaer is wary of the threat he still poses.

"Any goal he scores against us will be chalked off his goals tally," the United boss joked.

"He's a threat in and around the box and set plays. Wayne will show what he can do and he wants to prove there is still fight in the old dog.

"We've got to be on our toes, don't give him any space in midfield. He's got that Scholesy like passing range."