Messi has been sidelined by injury - which forced him to miss
Javier Mascherano and
Messi replied on 23 minutes after he was afforded too much space, but depsite the visitor dominating therefafter it was unable to find a winner.
Milan was gifted its 10th-minute lead after a defensive mix-up between Mascherano and Pique.
They failed to deal with a long punt forward before Mascherano's attempted clearance cannoned back off his team-mate and to the advancing Robinho.
He played a one-two with the overlapping Kaka, whose return pass left him with the time to pick his spot.
After failing to score in the league for the first time in two years during the weekend's 0-0 draw at Osasuna, the onus was on Barcelona to pick up its tempo.
It did so by controlling possession, but Milan retained its own threat through Robinho and Kaka while
Constant's eagerness to get forward was punished, however, on 23 minutes as Barca won the ball back high up the pitch and Messi was left in acres of space down the right flank.
With its lead lost Milan went close through Mexes, heading a Robinho corner wide, but Sanchez wasted the best chance before the break when he prodded Iniesta's lofted pass straight at Marco Amelia.
Amelia had to be alert after the break to deny Iniesta as Barcelona made a quick start after the break.
The goalkeeper also had to get down to deny Xavi while Messi hit a volley over.
Milan had seen little of the ball in the second period when Robinho missed a glorious chance to put it 2-1 up.
Sulley Muntari burst down the left and delivered a low cross that Robinho failed to get a touch on with the goal gaping.
Barcelona was the more likely scorer though - Riccardo Montolivo thwarting it on several occassions as he became overworked in midfield - with Adriano sliced wide after running onto a Messi pass.
A winning goal never arrived though to keep Barcelona top of Group H by two points from Milan.