Guendouzi had a chance to play in his teammate who was making a run down the channel. Instead, he got caught dwelling and decided to throw himself on top of the ball in order to try and win a free kick. (The referee wasn't having any of it.) A cynical habit of Guendouzi's and not one he's only recently developed.
Thankfully Saliba showed good recovery pace to deal with the situation -
There was another instance earlier in the first half where he overhit a simple pass and a counter attacking opportunity went begging. (Sampaoli was going berzerk on the touchline.) As usual, Douzi's work ethic was great, but his decision-making and awareness left a lot to be desired.
As for Saliba, he's had a convincing debut. Had one or two dodgy moments earlier in the half. One time he put the keeper under pressure and the other time he got dribbled a bit too easily. No major mistakes and he went from strength to strength as the game progressed. Without a doubt one of Marseille's standout performers.