Inter captain Javier Zanetti said he "cannot understand" how the reigning European champions twice gave up leads in a 5-2 home defeat to Schalke.
Dejan Stankovic had given Inter the lead inside the first 30 seconds with a long-range volley and, while Joel Matip levelled, Diego Milito then restored Inter's advantage from close range.
However, Inter's gameplan unravelled when Edu made it 2-2 just before the break and in the second half Raul stabbed home from close range to make it 3-2.
Andrea Ranocchia's own goal extended the visitors' advantage before Cristian Chivu was sent off. Edu then scored again with a shot from outside the box to leave Inter needing to score at least four goals in Germany to progress.
"I cannot understand what happened," Zanetti told Mediaset Premium. "I can't even explain it to myself, let alone to you.
"We had a good start, then there was a black-out. Now we have to stay calm, look ahead and begin again by thinking positive. We've got to keep going, think of Serie A and then we'll go to Germany to give it all we've got."
Raul, meanwhile, was delighted with the way Schalke performed on the night.
"We played a fantastic game, just the perfect performance," he said. "It's not often you see five goals scored away from home in Europe, especially on the turf of the reigning champions, but there are another 90 minutes to go.
"The difference was above all in the second half, when we were superior and brought home the win. We'll have to be very careful in the second leg, as eliminating Inter will not be easy."