Right-hander Alejandro Pena of the Los Angeles Dodgers will be out several months and might miss the 1985 season, according to Dr. Frank Jobe, team medical director, who also acknowledged damage to Pena's shoulder was so severe as to be career-threatening.

"He throws awfully hard. People who throw that hard can tear up the (shoulder) joint," said Jobe, who performed arthroscopy Tuesday. "He had worn off cartilage in his right shoulder and it had to be trimmed.

"At that time, we announced it would be two or three months before he's throwing, and we don't know if that's accurate. (The condition) can come back. More surgery is possible."

Pena, 25, was 12-6 last season with a National League-leading 2.48 earned run average.

New York Yankees: First baseman Don Mattingly, the 1984 American League batting champion, underwent arthroscopic surgery yesterday to repair a slight cartilage tear in his right knee.

Dr. John J. Bonamo diagnosed the injury, sustained in weight training, as a minor tear of the medial meniscus cartilage.

Mattingly, 23, hit .343 with 23 homers and 110 RBI last season. He can begin light exercise immediately and will report on schedule to training camp in Fort Lauderdale, Fla., on Tuesday.

Bonamo expects a three-week recovery period. Mattingly should be able to take batting practice after the second week.

There was an addition to the Yankees' 40-man roster, right-handed pitcher Al Williams, who signed a one-year contract.

Williams, 30, pitched the past five years with the Minnesota Twins, going 3-5 with a 5.77 ERA in 17 games last season.

Houston Astros: Shortstop Dickie Thon, who has had blurred vision since being hit by a pitch last April, began his comeback.

"I'm positive I'm going to play," Thon, 26, said. "I think I can get back to the way I was. Maybe better. That's how I have to look at it."

Thon's vision in his left eye was as poor as 20-150 after the beaning by the New York Mets' Mike Torrez. It's now 20-40, and he no longer suffers headaches.