But the Colombian proved his worth to the Rossoneri with an 85th-minute winner against Sampdoria, securing three points on the road despite his teammates' sluggish performance at the Luigi Ferraris.
The Blucerchiati dominated large spells of the match and had two goals disallowed. One was flagged offside, while the other was deemed a handball on feisty Sampdoria forward Luis Muriel.
Milan pounced on an error in the middle of the pitch moments later. Suso jumped onto the ball before splitting the defence with a perfect pass to Bacca for the clincher.
The biggest criticism against the 30-year-old this week was about his lack of movement. The idea was he couldn't create goals by himself.
But he showed Friday he's far from the problem on this team. A striker that thrives in the penalty area, he needs very few chances to give Milan a chance to win.
Bacca now shares the Serie A lead in scoring with four goals.
Montella opted to start summer signing Gianluca Lapadula in Bacca's place, although the Serie B scoring sensation did little to convince his manager in the 63 minutes in which he played. Lapadula made just 13 touches throughout the match as Milan struggled to create offence.
Sampdoria will feel aggrieved with result, having produced a good showing on home turf.
Gianluigi Donnarumma kept Milan in the match with acrobatic saves early in the first half.
The visitor slowly grew into the game, with M'Baye Niang testing the Sampdoria defence on the left flank and Suso weaving through the opposition.
When the moment of truth finally arrived, Bacca was there to finish it off.