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

The latest setback in a tumultuous season for the Yankees designated hitter occurred Tuesday night in Toronto. Giambi was examined in New York yesterday 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,” the Yankees general manager, Brian Cashman, said.

Although 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 yesterday’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.