Ratliff, who got tangled up with Charlie Ward of the Knicks as both fell out of bounds last Tuesday, had the foot immobilized in a boot Monday after an MRI revealed the stress fracture.
The Sixers brought in free-agent center Stanley Roberts for a workout and could sign him as early as Tuesday, general manager Billy King said.
"I know this is something you have to deal with at times," said Ratliff, who was hobbled by a thigh bruise in the playoffs last season and sat out the fourth and final game of the second-round series against Indiana. "I've been injured before, but never to this extent."
The 29-year-old Roberts is one of the most injury-prone big men in the NBA. He's had two ruptured Achilles' tendons, back surgery and is coming off shoulder surgery.
"I like the kid," coach Larry Brown said. "I want him in our league. If this is a good situation for him, good. But that's up to him."
Roberts, who said he weighs 315 pounds, went through a light workout Monday.
"Very impressive," King said. "On cardiovascular, he's not where he wants to be. But he's a great post-up player. He can score on the block."
Roberts had the best year of his career with the Clippers in 1991-92, when Brown was the coach. The Sixers tried to get him last year, but he signed with Houston. After Philadelphia, he has trips planned for Miami and Milwaukee.
"It depends on how things go here whether I'll complete that trip or not," said Roberts, who would get the veteran's minimum of $760,000.
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