Belotti was one of a number of players whose performance was below par as the Azzurri were resoundingly beaten at the Santiago Bernabeu, courtesy of two goals from Isco and one for Alvaro Morata.
The result saw Spain move three points ahead of Giampiero Ventura's side in their World Cup qualifying group with three games to play.
The 23-year-old explained his bizarre injury and went on to defend coach Ventura's decision to play an attacking 4-2-4 formation.
"I was stung by an insect overnight and my eye puffed up," Belotti said to Rai Sport.
"Our regrets are throughout the entire game. The way we prepared, with a very attacking line-up, was because we wanted to come here and get a result.
"Football is a team game, everyone has to contribute and work both on and off the ball. That is what the coach asks of us and every side in modern football has to defend with 11 men.
"Spain are a very attacking side, yet at times they had 11 men behind the ball. I think this match can be a real education, as we could have done better and unfortunately we didn't, but we can learn from our mistakes."