The defensive midfielder struck midway through the second half to decide a dour game in which both sides struggled to break down one another’s defense and create chances.
Juventus talked of derailing Milan’s progress to the title, but lacked ideas and a cutting edge. Coach Luigi Delneri eventually turned to the team’s talisman Alessandro Del Piero, but even he failed to spark any life.
Milan leads the Serie A by eight points from Inter Milan, which could close the gap to five by beating Genoa on Sunday.
The loss was Juventus’ seventh in its last 11 games.