Roger Clemens, the key to Boston's pennant hopes, will miss a start for the first time in three years because of a back problem, the Associated Press has learned.

A source close to the two-time Cy Young winner confirmed the Red Sox plan to rest Clemens for about 10 days. Instead of pitching against Oakland during the weekend, Clemens would face California early next week, possibly on Wednesday.Clemens, 15-8, endured the worst and shortest outing of his career Sunday against Detroit. He gave up eight runs in 1 1-3 innings and lost his third straight start for the first time ever.

After the game, team doctor Arthur Pappas said Clemens had a slight muscle strain in the left side of his back. Pappas said it was not known how Clemens hurt himself and that he would be given muscle relaxing medication.

"It's day-to-day," Pappas said Monday, an off-day for the Red Sox. "He won't go on the disabled list."

Boston manager Joe Morgan thought Clemens' next outing would be delayed, but not by much.

"I just have a hunch he may have to be pushed back a day or two," Morgan said. "I don't know for sure, but I have a feeling he might miss one."

But sources said Clemens would be held out of the rotation for at least a week. The target date for his next start is Aug. 24 at Fenway Park against the Angels.

Clemens taken his turn in the Boston rotation for every start since the 1986 season. He missed the final month of the 1985 season because of surgery on his right shoulder.

The Red Sox, trailing Detroit by three games in the American League East, need Clemens for the team to stay close in the stretch. Clemens has lost twice to the Tigers and once to Milwaukee in his last three appearances.

In his last 14 2-3 innings, Clemens has given up 18 runs and walked 11. Clemens, recognized for his control, had walked only 37 in his previous 197 innings.

Pappas said Clemens' back trouble is affecting his control but not the velocity on his fastball. Clemens has tried to adjust but has overcompensated, Pappas said.

Clemens leads the major leagues with 242 strikeouts and seven shutouts. He is sixth in the AL with a 2.86 earned run average.

Clemens went 24-4 and led Boston to the World Series in 1986 and was 20-9 last season.