Williams' voice shook with dejection and disbelief as she said she can't serve because of a problem with her right pectoral muscle.
Mouratoglou adds that there was a concern that if Williams tried to play Monday, she would risk making the injury worse and not be able to play at Wimbledon, which begins July 2.
There has been no love lost between the pair since and Monday's French Open meeting - their first since the 2016 Australian Open - has all the makings of a classic grudge match.
The American, 36, was playing in only her third tournament since giving birth to her first child last September, and her first Grand Slam.
Paris: Serena Williams admitted here Monday her dream of an eighth Wimbledon title could either be saved or shattered in a doctor's office here this week.
"I'm beyond disappointed. You know, I gave up so much, from time with my daughter to time with my family".
When Diego Schwartzman takes to Court Philippe Chatrier on Wednesday for his maiden Roland Garros quarter-final, he will be looking to accomplish what has often been described as the toughest feat in tennis: Defeating Rafael Nadal in a best-of-five match on clay. Williams has won 19 of 21 so far, including 18 in a row.
Two-time champion Sharapova was gifted a walkover when Williams pulled out injured ahead of their last-16 tie, but the Russian's Paris progress came to a crashing halt with a 6-2 6-1 defeat by Spain's 2016 victor Muguruza.
Next up for Sharapova is Garbine Muguruza, who also moved on due to a retirement.
"Well, I think it would be odd for me not to include someone that I have competed against for so many years", a glum Sharapova, who has lost 19 of her 21 matches against Williams, told reporters.
Sharapova also wished her speedy Recovery. Muguruza, who is 3-1 against the No. 1-seeded Halep in her career, has never squared off with her in a Grand Slam. And that's why it's been hard to play for her. Sharapova got a walkover into the quarter-finals as Serena Williams withdrew from the highly-anticipated match in less than an hour before the match. "Her game matches so well against mine".
The 2014 U.S. Open victor will now face another big server, Juan Martin del Potro, who won his own match of tall big servers, knocking out the only American left in the draw, John Isner, in straight sets, 6-4, 6-4, 6-4.
They had been knocked out on Sunday by Andreja Klepac and Maria Jose Martinez Sanchez with the American sisters getting a 6-0 bagel in the final set. She told a "shocked" media that she had suffered a pectoral muscle injury. Physically I'm doing great.
"I sacrificed so much to be at this event".
"I think my body will need a little bit of rest, but mentally I'm ready to go", said five-time major victor Sharapova. In his first grand slam fourth round, the free-swinging 22-year-old led Nadal by a break in the first and third sets. "But also, I made a promise to myself and to my coach and to my team that if I'm not at least 60 percent or 50 percent, then I probably shouldn't play".