NEW YORK - Jason Giambi will be sidelined at least three weeks and possibly far longer after tearing tissue in the arch of his left foot while rounding the bases on a home run.

The latest setback in a tumultuous season for the New York Yankees designated hitter occurred Tuesday night in Toronto. Giambi was examined in New York on Thursday by Dr. William Hamilton and will be placed on the disabled list before Friday’s game at Boston.

“I’d say it’s a severe injury,” Yankees general manager Brian Cashman said.

While the best-case scenario likely would have Giambi returning in three-to-six weeks, Cashman couldn’t rule out that Giambi could miss the remainder of the season.

“That I don’t know,” he said. “Ultimately it simply will be ready when it’s healed. Right now I can’t tell you how long that’s going to take.”

Giambi was shifted from first base to designated hitter this year and hit .322 with four homers and 17 RBIs in April. His foot began bothering him soon after he played the field for the first time on April 28, and he batted .117 in May with three homers and six RBIs.

He originally was diagnosed with a bone spur and switched to shoes with orthotics that he said relieved the pain. Giambi then hurt the foot more severely when he homered in the seventh inning Tuesday night.

“Rounding the bases he felt a pop,” Cashman said. “There wasn’t a tear before. Now there is.”

Giambi traveled to New York on Thursday’s day off, intending to get a cortisone shot, but Hamilton said Giambi had plantar fasciitis, inflamed tissue that causes pain near the heel, and a partially torn plantar fascia, connective tissue between the heel and that base of the toes that supports the arch of the foot.

Giambi’s foot will be put in a walking boot, which will be removed in three weeks. He will have another MRI exam then, but Cashman said the boot might go right back on.

The 36-year-old Giambi, the 2000 AL MVP with the Oakland Athletics, is in the sixth season of a $120 million, seven-year contract with the Yankees and has been in the news this season for his role in baseball’s steroids controversy.

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In the May 18 editions of USA Today he was quoted as saying, “I was wrong for doing that stuff,” which many interpreted as an admission of steroids use, and the Daily News reported five days later that he had failed an amphetamines test within the past year.

Giambi met last week with lawyers for Major League Baseball, and his case has been turned over to commissioner Bud Selig, who hasn’t said whether he will attempt to discipline Giambi.

Melky Cabrera is likely to receive more playing time in the outfield while Giambi is sidelined, with Johnny Damon, Bobby Abreu and Hideki Matsui seeing spells at DH. Kevin Thompson likely will be brought up from the minors.

“At this stage we’ll just reconfigure the roster and move forward,” Cashman said.

Giambi has been among the struggling hitters in New York’s lineup. The Yankees are just 22-29, tied for last in the AL East and 13½ games behind first-place Boston heading into a weekend series at Fenway Park.