The Frenchman, who finished second in the 2013 Drivers' Championship, said: "I'm really happy to be back on the grid again. It's really annoying to watch a race knowing you should have been part of that."
Asked how Mattson felt about being shoved aside for the Canadian GP, he said: "It was a big blow for me, and to be honest I still don't really understand the reasoning. I don't feel betrayed or anything, I just want to prove myself these next two races and then we will take it from there."
Team principal Andae d'Hazormé: "I'm very happy that Jean responded positively to our request to resume his role at KLM Team 23. I'm sure he will be very valuable to the team in the next two races."
Rumours has it that title sponsor Air France-KLM is unhappy with Team 23's management and the lack of results. There is a possibility that the team will lose their title sponsor mid-season.