It is not unusual for Rory McIlroy to hold off hitting golf balls at East Lake until Wednesday of Tour Championship week. After all, he has won the FedEx Cup title three times—including a year ago—and is more than familiar with the Atlanta layout. But when the four-time major winner wasn’t seen swinging a club on Wednesday, it was obvious something was up.

And indeed there is. McIlroy is suffering from a muscular injury in his lower back that he suffered while at home earlier this week, Australian Golf Digest has confirmed. While McIlroy sat for a press conference at East Lake on Wednesday, he didn’t hit his first golf balls here until Thursday morning, and only hit about 20 shots on the range. That was after having treatment on his back.

Despite the issue, McIlroy teed off at 1:49 p.m. with Masters champion Jon Rahm in the second-to-last pairing, albeit seemingly nursing some pain as he began the first round of the season finale. McIlroy is third in the FedEx Cup standings and with the staggered start, began the tournament with an adjusted score of seven under par, three behind leader Scottie Scheffler.

McIlroy is playing his 18th event of the year, 17 on the PGA Tour including the co-sanctioned Scottish Open and the DP World Tour’s Dubai Desert Classic in January, both European circuit events that he won.

After the Tour Championship, McIlroy is scheduled to travel to the U.K. for the Irish Open in two weeks and the DP World Tour’s flagship BMW PGA Championship at Wentworth. After a week off, he’ll spearhead the European team at the Ryder Cup in Rome late in September.