Santon says they lack "the right mentality" after their Champions League loss.
"I think Inter are a great squad, but we received a tough blow at Barcelona and must absolutely get back on track straight away," he told the Gazzetta dello Sport.
"It's a question of mentality. The team is there and very strong, too."
There has been controversy at San Siro with Mario Balotelli 'confessing' he is in fact a Milan supporter, so there is another heavy blow for the Nerazzurri faithful.
"When I was a kid, I also supported Milan. I was influenced by the presence of Marco Van Basten, who was my favourite player," added Santon.
"I think a player can support any side he likes, as long as he defends the club he represents. Who knows how many Inter players are Milan fans and vice versa. We remain professionals."