Villa won 2-1 thanks to a first-half brace from Donyell Malen. The visitors' goal didn't arrive until the 90th minute through Joel Monteiro.
Seoane lamented afterwards: "We completely missed the beginning of the game. We were not aggressive enough or quick enough to make the right decisions in defence. We defended poorly in the first half, and left ourselves too much to do.
"The plan (after halftime) was to be more competitive. We made changes, adapted the system to create more man-to-man situations defensively.
"We were braver with the ball, showing the quality we have, and we were able to play our game. But it was too late."
Young Boys skipper Loris Benito also stated: "It was hard but we knew it was going to be against such a great team. We set out to frustrate them but we conceded twice so we had to approach it differently.
"In the second half we played with a back four and I have to say we are much more used to it. Aston Villa also maybe dropped down a gear so we were more comfortable and, in the end, got a goal back."