United were well below par as Kelechi Iheanacho’s double fired Leicester to into their first FA Cup semi-final in 39 years.
Youri Tielemans also netted to put the Foxes – four-time runners-up – a game away from their first FA Cup final since 1969.
Mason Greenwood did haul United level in the first half after Fred’s mistake presented Iheanacho with the opener, though Tielemans and Iheanacho went on to secure just Leicester’s second win over United in 23 years – but a deserved one as they outplayed and outfought the visitors.
The Europa League is now United’s only realistic chance of a trophy and Solskjaer, who made five changes from Thursday’s win at AC Milan, had no regrets over his team selection having left talisman Fernandes on the bench until the second half.
He said: “I would have started the same team again. Every team selection has reasons behind it. On Thursday Bruno broke all his records physically and the boy is not inhuman.
“He has played a game every three or four days. It was a chance to start both Donny (Van de Beek) and Paul (Pogba) but the accumulation of games maybe caught up with us.
“We knew we had substitutes who could come on and make a difference Everyone could see we weren’t as sharp as we have been.
“You are always disappointed, especially when it’s the end of the line of a cup run. We couldn’t find the normal spark, brightness and energy.
“Maybe all the games caught up with us. Thursday was a big night and today we didn’t have the zip.”
Additional reporting by PA.