United won the game 4-0, following a brace from Rooney and one each from Park and Fletcher.
Galliani knew they would lose due to their injury problems.
“I knew it was going to go badly yesterday afternoon. We had taken Alexandre Pato, Alessandro Nesta and Luca Antonini in the squad, but all three of them pulled out with injuries on Tuesday,” he said.
“Without such important players and taking on a Manchester United side of such quality, it was always going to be extremely difficult.
“I don’t think it will affect our Serie A form. Leonardo already did very well in the locker room to reassure the players.”