SAN DIEGO — Rafael Nadal, the 21-time Grand Slam tournament singles champion, announced Tuesday that he had a stress fracture in his rib cage and would not play for “four to six weeks.”

Such injuries can be slow to heal and difficult to treat. Nadal, 35 and back up to No. 3 in the ATP Tour rankings after an outstanding start to the season, appears certain to miss a significant chunk of the clay-court season, including next month’s Monte-Carlo Masters and Barcelona Open.

Nadal is the most successful clay-court player in history, and 13 of his record 21 major titles have come at the French Open, which begins May 22.

“This is not good news, and I did not expect this,” Nadal said in Spanish on Tuesday in a social media post. “I am downbeat and sad because after such a great start to the season, I was coming to a very important part of the year with very good feelings and results. But hey, I have always had this spirit to fight and to overcome and what I will do is have patience and work hard during my recovery.”

Nadal said he injured the rib during his grueling three-set victory on Saturday in the semifinals of the BNP Paribas Open over Carlos Alcaraz, his 18-year-old Spanish compatriot. Nadal struggled to breathe without pain during the latter stages of that match, played in blustery conditions in Indian Wells, Calif. He was treated on the court but, still unclear on the severity of his injury, he chose to play in Sunday’s final against Taylor Fritz of the United States.

Fritz, 24, defeated Nadal 6-3, 7-6 (5) to win the most significant title of his career. Nadal was clearly diminished: serving at slower speeds than usual. He took a medical timeout off court after the first set and was treated on the court late in the second, wincing as he lay on his stomach during a changeover and a tour trainer worked on him. But Nadal still came within two points of forcing a decisive third set.

He underwent medical tests upon returning to Spain on Monday and is now back in an all-too-familiar mode: rehabilitating an injury. Nadal has dealt with injuries throughout his remarkable career and missed most of the second half of the 2021 season because of a chronic foot condition after losing to Novak Djokovic in the semifinals of the French Open. He caught the coronavirus in December but recovered quickly and won his first 20 singles matches of the season, including the Australian Open title.

But this is a new type of injury for Nadal, one that could recur and, according to the retired orthopedic surgeon Bill Mallon, could require longer than four to six weeks to heal. “You can’t put your rib in a cast,” Mallon said.

Treatment options are limited, though some patients use bone stimulator devices, which use electrical or ultrasonic impulses to try to speed healing, said Nicholas DiNubile, an American orthopedic surgeon. Nadal has never played in the French Open without competing in a preliminary event on clay. But it could be a race against the clock to compete in early May in the Madrid Open or Italian Open.

Before announcing the injury, Nadal had withdrawn from this week’s Miami Open, a Masters 1000 event on a hardcourt. Djokovic, back at No. 1, will also miss it because of the United States’ travel ban on unvaccinated foreigners. But Fritz, the rising American star who played Sunday’s final despite a right ankle injury, is expected to play.

“With treatment and a late start, he should be good to go,” Paul Annacone, one of his coaches, said Tuesday.