La Dea had been 3-0 up within 21 minutes thanks to Josip Ilicic, Robin Gosens and Luis Muriel goals, but the Granata clawed it back to 3-2 by half-time and eventually equalised with a Federico Bonazzoli header.
“Some players switched off, some didn't, but the last 10 minutes of the first half it really looked as if we had just stopped. Torino had four attempts before finally scoring the second goal," said Gasperini.
“I think the underlying cause of it all was fatigue. We arrived late from Naples (after the Coppa Italia semi-final midweek), some hadn't fully recovered and we were a little limited.
“Hateboer is going to be difficult, but we hope to have Maehle back for the second leg with Napoli. It's strange, we went 3-0 up after 21 minutes and then our levels just plummeted."
He added: "We are always using the same defenders and that can lead to fatigue in some areas of the team. There was a period where we conceded very few goals and it was difficult even to see chances against us. We made a lot of errors today in situations that were easy enough to control.
"I honestly don't know how it was 3-2 at half-time when Torino barely had a shot on target. That lifted Toro's spirits, we were relatively in control of the second half, but then a set play can always make the difference when the result is open."