Zlatan Ibrahimovic had not played since the first half of Milan's match against Juventus on January 23.
He has since been missing due to an injured heel.
ARCHIVE: Here Zlatan was replaced with injury (January 24):
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Zlatan Ibrahimovic in a duel.
Photo: Bildbyrån
But tonight, when Milan (third before the match) played away against Napoli (who were second) he was back.
Giroud match winner
The 40-year-old Swedish star started on the bench, but had to jump in after 89 minutes of play.
Then Milan led 1-0, after Olivier Giroud four minutes into the second half poked a shot from team captain Davide Calabria - past Napoli goalkeeper David Ospina.
Maignan kept a clean sheet
Napoli then had quite a lot of ball, but difficult to create really dangerous chances.
And the ones they created well saved Milan's goalkeeper Mike Maignan without much trouble.
Milan's victory means that they take over the league lead.
They are two points ahead of Inter, but have one match more played.
That Zlatan Ibrahimovic is back is probably good news also for Sweden's national team captain Janne Andersson.
There are only 18 days left before his national team meets the Czech Republic, in the playoff semifinals of the Qatar World Cup.