Roma won the second-leg of their semifinal 3-2, though lost the tie 8-5 on aggregate.
“I think we had a good first half in Manchester, so we lost the round, but did well over the two legs otherwise," the coach told Sky Italia.
“I thought it was possible for us to reach the Final, even when we were paired with Manchester. It was possible, and we were in front at half-time at Old Trafford, but we got it wrong in the second half.
“It became very difficult to win 4-0 here, but we gave it our best and scored three goals this evening. Unfortunately, Manchester United made the most of all their chances."
Fonseca gave 19-year-old Ebrima Darboe a chance off the bench, only his second professional appearance in the Roma jersey.
“Ebrima had been training with us for some time and when Chris Smalling was injured, I looked to him. As soon as he came on, he looked like a seasoned professional, so he did a wonderful job and so did Zalewski."