Andrea Cambiaso and Mateo Retegui had Italy 2-0 ahead in Rome before Lorenzo Pellegrini's red card eventually saw Belgium fight back through goals from Maxim De Cuyper and Leandro Trossard.
"There are incidents that can throw everything you prepared and did out the window,” said Spalletti in his press conference.
“We must be better at reacting to these situations and directing those incidents, but at times like this evening it was not possible. I saw the disappointment of the lads in the locker room because they didn’t win the game. You could see they were aware that they were in control and on track for another victory, but there was also a bit of bad luck.
“It was a good performance even down to 10 men, though we got a bit too deep. I did have doubts at half-time whether to go back to a four-man defence, but I thought if we held out for 20 more minutes and then switched, it would make it more difficult for them.
“It’s tough against the Belgium forwards, because they have incredible pace, so they punt the ball 10 metres ahead and know they will run onto it first.”
He added, “I think we actually improved, as instead of conceding in the first minute, this time we scored. Everyone was ready to sacrifice themselves, we passed our way out from the back and attacked the spaces. We were forced to do a bit more when down to 10 men and had some opportunities when winning the ball back to score a third goal, whereas we allowed Belgium practically nothing from open play against Donnarumma.”