The former Bordeaux stopper was replaced by Charles Kabore after just five minutes of action at the Stade Velodrome, following a collision with opposition attacker Esmael Goncalves.
A sprained right knee was the diagnosis, but more details on Diawara's condition will be given within the next 48 hours.
However, the worst case scenario would see the Senegal international miss Tuesday's Champions League quarter-final first-leg with Bayern Munich.
Marseille saw their run of five consecutive defeats in Ligue 1 come to an end against Nice, but are still without a league win since February 26, when they beat Brest 1-0.
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