The former Manchester United full-back tapped home Achraf Hakimi’s cross with 14 minutes remaining to seal an 11th successive victory.
Strugglers Cagliari had frustrated Inter who saw defender Stefan de Vrij head against the bar after 68 minutes.
Antonio Conte’s side now require 16 points from eight games to win their first league title since 2009-10.
AC Milan are second, with Juventus – who have won the past nine titles – one point behind them after a 3-1 win over Genoa.
Dejan Kulusevski gave Andrea Pirlo’s side the lead with a clever finish and Alvaro Morata made it 2-0 after Cristiano Ronaldo hit the post.
Gianluca Scamacca headed in for Genoa but Weston McKennie slotted home to make the game safe.