The former Newcastle and Manchester City man started the game before being replaced after 71 minutes and admits he is still some distance from full form and fitness.
"I still have a long way to go," Barton told L'Equipe. "I have to make a lot of improvement, but it is logical as I didn't make any preparation.
"When we scored the equaliser, I stood up from the bench to celebrate, but I had cramp, it was terrible!
"I am very proud that I have been able to make my debut for Marseille.
"This is really a very good team, a real group playing together. The team has ambition and play great football.
"It was a tough game against a very good team in a stadium where the atmosphere was really fantastic.
"The mood of the squad is impressive and we managed to come away with a draw."