Kaka was relatively quiet in what could be his last game for the seven-times European champions.
Milan fans displayed banners such as "Kaka is Priceless" and "Hands Off Kaka" and sang "Don't Sell Kaka" after the club admitted this week they are considering an offer from Manchester City that is reportedly worth over 100 million euros.
Coach Carlo Ancelotti said he had not given up hope of keeping Kaka.
"Yes, I hope (he stays) because he's an important player for us," Ancelotti told Sky television.
"The situation is still developing, anything can happen. It's true that there's been an important offer for an important player, but time is needed to do all the evaluations.
"He experienced very positive emotions (at the match), with a whole (group of) people showing him great affection."
Pato drilled in with the help of the far post in the seventh minute to give third-placed Milan 37 points from 19 games, six behind Serie A leaders Inter Milan, who visit Atalanta on Sunday.
Loan signing David Beckham had a mixed home debut for Milan, who had Marek Jankulovski sent off in stoppage time for a second yellow card.
The Englishman was booked and misplaced a few passes but he also recovered a ricochet in the move that led to the goal and had a decent shot pushed away for a corner just before halftime.
"He was tidy, very disciplined," Ancelotti said. "He can't yet play for 90 minutes, he felt Fiorentina's pressure a bit in the second half. Apart from the tactical discipline, what I like about him above all is that he has a very low error rate."
The hosts had a scare a minute before Pato notched his fifth goal in three league games when the referee awarded a penalty for a challenge on Fiorentina's Stevan Jovetic, but he overruled it when the linesman told him keeper Christian Abbiati got the ball.
With Ronaldinho on the bench for most of the match, Ancelotti's men faded as the first half progressed.
Montenegro midfielder Jovetic came close to equalising with a powerful shot, while Riccardo Montolivo had a long-distance strike well saved and Alberto Gilardino headed over from a good position.
Kaka's best moment was just before the half hour when he went on a surging run from his own half that ended with him going down in the area and the referee letting play continue.
The home team had a good spell of pressure after the break only to gradually surrender control again. Abbiati had to produce a stunning save to deny Mario Alberto Santana, who also hit a shot narrowly wide five minutes later.
Earlier, Siena beat second-from-bottom Reggina 1-0 to end a run of four consecutive league defeats thanks to substitute Mario Frick's 75th-minute goal.
Second-placed Juventus, who trail Inter by four points, visit Lazio in Sunday's late game.
