The Spanish outfit beat Lyon 4-0 in the playoff round to advance into the group stages, but finished with just one point after losing five of their six matches.
"We gave everything we had throughout the group but, for the moment, it's a level a little above where we are," Elustondo told reporters at a press conference.
"I'll take away the impression that our rivals have been really intense in their play and perhaps superior physically. The experience will do us nothing but good in terms of learning and developing."
Manchester United topped Group A, while Bayer Leverkusen finished second.