Ole Gunnar Solskjaer blamed United’s FA Cup defeat to Leicester on their busy schedule and their midweek trip to Milan.
The Red Devils were well off the pace as their FA Cup hopes were crushed by the Foxes.
He told the BBC: “We didn’t have the spark tonight – but it’s understandable. This team have been fantastic in the last three or four months.
“We played every three days and been on a great run. It just caught up with us, all the games and travels.
“Thursday night in Milan was a big night and took a lot of out of us physically. We didn’t have the extra zip, authority and confidence today.
“We tried to get on the front foot and start well because sometimes you can get that adrenaline boost and confidence if you get a goal and that can carry you over the line.
“We’ve got too many players that have played too many games and too many that haven’t really had much football, Anthony Martial, Donny van de Beek, Paul Pogba and others are trying to get in who haven’t had enough football. We didn’t have enough against a tough team.”