Walter Mazzarri's side has been inconsistent throughout this season and still has work to do if it is to secure qualification for the Europa League, with Parma and Lazio both hot on the heels of the Nerazzurri, who are in fifth place.
A winless run of four games has come at a poor time for the Milanese side, but Guarin, who is committed to the club until the end of the 2016-17 season after signing a new contract in March, is remaining positive.
"I have agreed a deal for another three years. I hope to sign another contract once that one's up," he told reporters. "I have not signed thinking that I will leave. I feel good here. It's been a difficult year, but we have to face up to those problems if we want to achieve our goals.
"We absolutely need to win the next game [against Sampdoria on Sunday]. So far we haven't been lucky in any sense this season, but we're looking forward to this weekend. We, the players, just have to think about the next game and then the next one after that. Sunday is fundamental for us. We simply have to win."