Andrea Stramaccioni slammed his Inter side's performance during their 4-1 loss to Fiorentina in Serie A on Sunday as "too ugly to be true."
The Nerazzurri were convincingly beaten by the Florentines at the Artemio Franchi, with Adem Ljajic and Stevan Jovetic both netting doubles to leave their Scudetto hopes in tatters.
The 37-year-old believes his troops looked jaded after their Europa League exertions in midweek, but admitted that their efforts were well below his expectations.
"I think it's all too easy to read this match, aside from Fiorentina's merits," he told reporters.
"We could have done without this defeat, but I don't know how we can play so differently from Thursday to Sunday. Maybe I made a mistake. Inter were too ugly to be true.
"We played two games with more or less the same players, but I never imagined a game like this. I saw my [team] were really tired and struggling.
"I thought we would pay a small price for the fatigue, but not to this degree. I evaluated the situation incorrectly."
The defeat leaves them on 43 points from 25 games, 12 points behind league leaders Juventus.