The referee pointed to the spot on the half-hour when Isla stuck out an arm when blocking Antonio Nocerino’s header, though replays suggest the ball actually hit him on the ribs.
Robinho, making his first start in Milan colours since the opening match of the season, took responsibilities for the spot kick.
The Brazilian squeezed his shot into the bottom left corner, despite Buffon getting a strong hand to the attempt, to score his first goal of the season for the Rossoneri.
Gianluigi Buffon almost gifted the hosts a second goal early in the second half when his weak pass went straight to Stephan El Shaarawy, but the forward was dispossessed by Andrea Barzagli before he could get his shot away.
Juventus pressed for an equaliser in the second half and came close with a number of chances, including Sebastian Giovinco’s spectacular overhead kick.
However, the Bianconeri were unable to break down a resistant Milan and consequently suffered their second defeat this term and their first away from home for 18 months.
The reigning champions could have gone seven points clear with a win, but remain just four ahead of Fiorentina in second and Inter in third, whilst Milan move up to ninth.
