The hosts failed to build on Lucas Biglia's opener and let Milan back in the game, as Montella's side salvaged a point at Stadio Olimpico
Suso's late strike salvaged a 1-1 draw for AC Milan against Lazio, who missed out on the opportunity to go fourth in Serie A.
A victory would have lifted Simone Inzaghi's side above Atalanta and Inter to within five points of Napoli, who occupy the final Champions League qualification berth, but Suso's 85th-minute goal earned Milan a share of the spoils.
Milan had ended a run of four straight defeats in all competitions with a 1-0 win at Bologna last time out - despite having two men sent off - and although they kept their full allocation on the field at Stadio Olimpico, they created little to trouble the hosts for much of this encounter.
Lazio took the lead in first-half stoppage time when Milan goalkeeper Gianluigi Donnarumma needlessly felled Ciro Immobile inside the area and Lucas Biglia smashed in the resulting spot-kick to Donnarumma's right.
The home side lost keeper Federico Marchetti to a reported knee injury in the warm-up and back-up Thomas Strakosha, having had little to do, was beaten late in the piece as Suso weaved between three Lazio defenders inside the box and placed a shot into the bottom-left corner.
Vincenzo Montella's side remain seventh, four points adrift of the Europa League places and one point ahead of Fiorentina, whom they host on Sunday.
Monday's outing began positively enough for Milan, who forced a succession of corners in the opening 10 minutes, but it was Lazio who threatened first - Biglia stinging the palms of Donnarumma before Immobile headed wide from Marco Parolo's cross.
The hosts maintained the pressure and Balde Keita twice brought comfortable saves out of Donnarumma either side of calls for a penalty when Immobile went down under a challenge from Gustavo Gomez.
Wesley Hoedt was next to go close for Lazio as he volleyed straight at Donnarumma from close range following a corner, Gomez relieved to see the ball trickle wide after smashing his subsequent clearance into the chest of the Dutchman.
Lazio could not afford to be complacent, though, and Lucas Ocampos provided a reminder of Milan's threat with a low shot on the break that was saved by Strakosha.
That was a rare foray forward, though, and the visitors were behind in first-half stoppage time as Biglia thumped home his penalty following Donnarumma's foul on Immobile.
Felipe Anderson fired narrowly wide in the 54th minute and Milan felt they should have had a penalty of their own midway through the second half when Ignazio Abate was bundled over by substitute Senad Lulic.
Montella opted not to bring top scorer Carlos Bacca off the bench but it was another substitute, Jose Sosa, who won the ball back and provided Suso with the chance to make his mark and earn Milan a point.