NEW YORK -- New York Yankees pitching coach Mel Stottlemyre left the team Monday, as planned, to undergo a stem-cell transplant and will likely miss the rest of the season.

Stottlemyre, 58, who was diagnosed with multiple myeloma a year ago during spring training, entered Memorial Sloan-Kettering Hospital to prepare for the procedure this week. It was not known what day the surgery would take place.

Stottlemyre already had left the team twice this season for scheduled treatments.

Former pitching coach and vice president of player personnel Billy Connors has been with the Yankees since Stottlemyre's last absence in August. He will take over as pitching coach for the remainder of the season for the two-time World Series champions, who lead the AL East.

Connors was with the team on its recent road trip to Kansas City and Boston -- but not in uniform -- to help ensure an easy transition for the pitching staff.

"This was planned," Yankees manager Joe Torre said before Monday night's game against Boston. "That's why Billy's been around, to be sure that when this day came nobody new showed up."

Stottlemyre, a three-time 20-game winner, is the only pitching coach Torre has had since coming to New York in 1996. But Connors has played a vital role working with pitchers at the Yankees' complex in Tampa, Fla.

Roger Clemens is 8-0 since returning from the disabled list. He spent time with Connors in Tampa as did David Cone, who went to Florida to work on his mechanics during his slump.

"Billy's been here watching Mel, listening to Mel," reliever Jason Grimsley said. "Billy's good at what he does. The important thing is taking care of Mel now."

Stottlemyre, who purposely didn't tell any of the players that he'd be leaving the team this week, was upbeat with starter Andy Pettitte this past weekend in Boston. Pettitte is exceptionally close to Stottlemyre.

"As far as the starting staff goes, everything's been good. It made Mel feel a lot better about going," Pettitte said.

Multiple myeloma is a type of cancer formed by malignant plasma cells. Normal plasma cells are crucial to the immune system, but when they are produced at an abnormal rate they can cause tumors, anemia and fatigue.

Roger Neilson, former coach of the NHL's Philadelphia Flyers, underwent treatment for the same disease.

Stem-cell transplant is a procedure in which certain blood cells critical to healthy bone marrow are harvested from the patient's blood, then reintroduced after the chemotherapy has destroyed most of the cancerous plasma cells.

Stottlemyre told Pettitte he'd be back sooner than expected.

"If anybody can bounce back, he'll be able to," Pettitte said.