Milan lost 1-0 via Victor Boniface's winner for Bayer.
But Fonseca remarked afterwards: "It’s true that we didn’t win, but it was the performance I liked the most since I arrived here.
“Above all in the second half, we had a fantastic performance, we created so many chances and just failed to convert them, which is not easy against a team like Bayer.
“We were sad in the dressing room, but we must also be balanced and recognise what we did. I am not satisfied with the result, but I am satisfied with the approach.”
He also stated: “Bayer have a very strong positional game, the first half was difficult for us and we closed down the central spaces, but they created problems combining primarily on the right flank. Within those difficulties, we defended well.
“It’s always the same problem people are trying to create.
"I realise that it’s tough for Rafa to track back, but I think the other side had more problems. Leverkusen didn’t find the central spaces, so they moved it down the wings and that is one of their strengths.
“It’s true that we need to give more support to our full-backs in general.”