Ronaldo has now scored eight goals in his last five matches.
"Ciro's a leader, scoring a very important goal in a hard match", said coach Simone Inzaghi, whose side host Zenit Saint Petersburg in the Champions League next week. Third-place Sassuolo is a point further back ahead of its match against Hellas Verona.
"Technically it was not a good performance but there are no easy matches and the team did what they had to do", said Gasperini, whose side failed to score for the first time this season. Today we had important players out like Sergej Milinkovic-Savic. those who replaced them gave their best.
Atalanta and Lazio are among four clubs on 14 points, three fewer than Milan.
Elsewhere on Saturday, Atalanta was held to a 0-0 draw at Spezia and Lazio beat bottom side Crotone 2-0.
Federico Bernardeschi had an early goal ruled offside before Ronaldo struck twice in a four-minute spell before the break.
The Portuguese striker blasted in an Alvaro Morata cross on 38 minutes, before latching onto a Merih Demiral header from a corner for the second from close range minutes later.
Juventus had chances to extend its advantage in the second half, notably when Demiral headed off the crossbar.
The Italy global was also involved in the second goal, sending Joaquin Correa through just before the hour mark as the rain briefly subsided.
It was Immobile's first match in almost three weeks after being ruled out with COVID-19, which also kept him out of Italy's three matches.
Correa doubled Lazio's lead in the 58th with a shot from a tight angle after dribbling past Crotone defender Pedro Pereira and going nearly to the byline.
Gian Piero Gasperini's Atalanta stalled before their big match at Anfield in four days' time, being held to a second consecutive stalemate.
Spezia had won only one of its last five matches but nearly took a surprise lead within the first two minutes when Diego Farias' curled effort smacked off the far post.
Diego Farias hit the woodwork for Spezia with Atalanta's Duvan Zapata denied by the post before the break.
The visitors nearly won the match late on but Spezia goalkeeper Ivan Provedel pulled off a stunning double save to deny first Mario Pasalic and then Robin Gosens.