Gonzalo Higuain's first AC Milan goal salvaged a 1-1 draw at Cagliari but Gennaro Gattuso's men are already falling off the Serie A pace.
Making his first appearance after serving a doping ban, Joao Pedro gave Cagliari an early lead when converting a rebound that Gianluigi Donnarumma perhaps should have saved.
Giacomo Bonaventura missed two great first-half chances to equalise but Higuain hit the target at Sardegna Arena on Sunday, in the process getting off the mark following his move from Juventus.
However, having thrown away a lead to lose at Napoli after their opener against Genoa was postponed, Milan languish in the bottom half after taking only four points from their first three games of the season.
Cagliari took the lead inside four minutes, Donnarumma tipping a drive from Leonardo Pavoletti on to the woodwork with the goalkeeper unable to keep out the rebound strike from Joao Pedro.
Milan were clinging on and the hosts soon hit the post again, Nicolo Barella's superb swerving strike deserving better, then Bonaventura planted a free header wide at the other end.
Bonaventura produced an even worse miss in the 27th minute, arriving at the back post to meet Davide Calabria's inswinging cross but sending his first-time effort flying over the crossbar.
Higuain was barely involved throughout the first half but levelled the scores 10 minutes after the break.
Franck Kessie charged down a defensive clearance and Higuain pounced on the loose ball, rounding goalkeeper Alessio Cragno easily before slotting a neat finish into the open goal from a tight angle.
Cragna did twice deny Suso and Higuain headed Diego Laxalt's cross wide as Milan pressed for a late winner, but the Rossoneri could not find a way through.
What does it mean: Scudetto bid failing
Milan have a game in hand but they already sit eight points behind leaders Juventus, who are seeking an eighth straight Serie A title. Challenging the Bianconeri already looks beyond Milan and their targets for the season may need to be reassessed - Champions League qualification could be a more realistic goal.
Pat on the back: Higuain shows his worth
Higuain barely had a kick before the break and was isolated as Milan's lone striker. But he showed his predatory instincts to haul Milan level, scoring what will surely be the first of many goals for his new club.
Boot up the backside: Bonaventura wasteful
During the first half, Bonaventura twice got into excellent positions to equalise for Milan. Yet he missed both chances, wasting the type of opportunities a poacher such as Higuain surely would have snaffled.
Key Opta stats:
3 - Joao Pedro has scored with his third touch in Serie A this season. Return. #CAGMIL— OptaPaolo (@OptaPaolo) September 16, 2018
- Milan have conceded at least one goal in their last nine Serie A games (W4 D3 L2).
- Joao Pedro scored with his third touch in Serie A this season.
- Cagliari scored their fastest Serie A goal since October 2016 (against Fiorentina).
- Gonzalo Higuain has scored seven goals in eight Serie A games against Cagliari.
- Cagliari hit the woodwork twice in a single Serie A game for the first time since January 2017 (against Genoa).
Milan travel to Luxembourg to take on Dudelange in their first Europa League group game on Thursday before hosting Atalanta in Serie A three days later. Cagliari are at Parma on Saturday.