Marseille were forced to settle for a 1-1 draw on Friday after Strasbourg grabbed an injury-time equaliser as France's Ligue 1 resumed after its winter break.
Undefeated at their Stade Velodrome home this season, Marseille went in front after just three minutes through a header from skipper Samuel Gigot.
However, 18-year-old Jeremy Sebas ensured a point for Strasbourg with a goal in the second minute of stoppage time.
"Sebas has enormous qualities, but we have to use him little by little. I am very happy for him, scoring his first goal at the Velodrome, it's difficult to do better than that," said coach Patrick Vieira.
He added: "Collectively, we performed very well. We remained united despite the goal conceded and with a little more success, we could have won."
Marseille moved into fifth place in the table, three points behind Brest while Strasbourg remain in ninth spot.