The 36-year-old Williams, who has not played on clay this season, withdrew from the Madrid and Rome tournaments because she was not ready to compete.
"Serena will play the French Open to win it. Can she do it? Serena can achieve anything - after being her coach for six years, I'm even more sure of that statement," Mouratoglou said in an interview with tour organizers WTA on Monday.
The Frenchman, however, admitted she returned too soon after her pregnancy, which resulted in early exits in Indian Wells and Miami in March.
"After her pregnancy, Serena had to rebuild her body. When she arrived at the Mouratoglou Tennis Academy, around April 30, we realised that she was not ready yet," said Mouratoglou.
"The time she lost after the delivery, with all the medical issues she had to go through, was missing."
The French Open starts on May 27.
(Reporting by Julien Pretot; Editing by Christian Radnedge)
