Ole Gunnar Solskjaer has suggested Barcelona's first-team players might have been knocked out of their stride after being rested for their La Liga game at the weekend.
Ernesto Valverde made 10 changes to his starting XI for Barcelona's trip to Huesca on Saturday, with only Marc-Andre ter Stegen keeping his place from the 11 that lined up at Old Trafford last Wednesday.
With United desperately scrambling for a top-four spot in the Premier League, Solskjaer could afford himself no such luxury as he fielded a strong side for the Reds' 2-1 win against West Ham.
Chris Smalling, Fred, Paul Pogba, Diogo Dalot and Romelu Lukaku all started against both Barca and West Ham, while Ashley Young and Luke Shaw only dropped out at the weekend through suspension.
"Having a midweek rest might just mess you up in terms of rhythm," Solskjaer told the United app.
"For our players - it is game against Barcelona, recovery, game against West Ham, recovery, and game again. They (Barcelona) had Saturday off and I don't know what they did, but it might have just changed their rhythm.
"You always look for positives and our players have got 90 minutes of hard work against West Ham and the confidence boost of a win, which is never to be underrated."
On the lessons he learnt from the first leg, Solskjaer added: "We need more quality in the last third, with crosses and finishing.
"Maybe we need more belief in what we did - we had too much respect in the first 15 minutes and never really won the ball off them.
"Even though we had a couple of chances - Rashy had his free-kick that went on wide, Paul just played Rashy straight through on goal, so we were close to one or two nil before they scored."
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