Rafael Nadal has been ruled out for between four and six weeks with a stress fracture of the rib.
The 21-time Grand Slam champion found breathing painful and said he was dizzy as he lost the final to Taylor Fritz.
Nadal, 35, is expected to miss both of next month’s clay court events in Monte Carlo and Barcelona, while the French Open begins in Paris on 22 May.
Nadal has won the title at Roland Garros a record 13 times.
In a post on social media, he said when he returned to Spain he “immediately went to visit my medical team to do tests after I played with discomfort in the final”.
He added: “As it turns out, I have a stress crack in one of my ribs. This is not good news and I did not expect this.”
Nadal has been the form player in the world this year, headlined by his victory in the Australian Open when he beat Daniil Medvedev in an epic final.
His defeat against Fritz in California was his first of the year, bringing a 20-match winning run to an end.
Nadal is not expected to be able to start practising again for four weeks and his next tournament is set to be the Madrid Open in six weeks.