Riccardo Orsolini’s 94th minute winner means Inter Milan are now level on points with Antonio Conte’s Napoli in the race for the Serie A title.
The two sides both sit on 71 points from their 33 league games although Inter remain at the summit with a goal difference of +40, 13 more than that of Napoli.
Barella, 28, refused to dwell on the defeat, however, and remains confident his side will end the season well.
“A defeat is always hard to swallow,” Barella told Sky Sport Italia.
“We’ve got a lot of fixtures and there is fatigue, but that is not an excuse. It is not normal that a team like ours concedes such a goal when it’s seven against three in the penalty area. We’ve got to do better, football is unforgiving.”
Treble dreams still alive for Inter Milan and Nicolo Barella
Inter remain in contention to win a remarkable treble as they prepare to face Barcelona in the Champions League semi-final, and arch-rivals AC Milan in the Coppa Italia semi-final on Wednesday.
“We must never feel that we’ve already arrived, so we always have something that we can do better,” Barella continued.
“We’re fighting on three fronts, we are giving our all, and these setbacks are painful. It was a balanced match, and a draw would’ve been a fairer result. Nothing is over yet, we are still top of the table alongside Napoli.
“It is going to be a tough battle, but we want to fight on all fronts and certainly won’t stop here.”