The Premier League side are experiencing their first taste of Europa League football, which will stretch their squad throughout the season.
After a battling 1-1 draw with Burnley, they must get ready for Thursday night's game against the Italian side.
Nuno said to the club's official website: "It's tough, but we got the reaction today, a good reaction, the fans felt that the boys gave everything they had.
"The boys worked very hard. The way the boys fought, the way the boys gave everything they had, the patience that they had, when they are tired the best decisions are always difficult to make, but it's something that we have to improve on – playing with tiredness and managing the game.
"It's a tough cycle – we have to go again Thursday. We have to recover and go again, but we embrace everything, but the performance, of course, was not the best one."