Pep Guardiola's side lead 3-0 at the break after embarrassing United in the first-half at Old Trafford.
Speaking after the game, Solskjaer said: "From their first goal and especially the second we struggled to get to grips with them. A good reaction second half, but first half until they scored it is was back and forth. We didn't cope with the setback well enough.
"We didn't deal with their system well enough, we know they can play that way. They played that way last season and beat Chelsea 5-0.
"First goal there is nothing we can do, second is sloppy and the third we just didn't recover.
"With Matic and his experience (coming on in the second half) he had more presence. It was more about wanting the ball and believing, passing the ball. Someone needed to take responsibility. We've shown before we have been down from a home tie and turned it around. PSG is the latest example and we have to believe that we can put on a performance.
"It was a decision late on on who was fit, it was no excuse. We have the players we have. We put a team out believing we could get a result."