LONDON -- Vikings quarterback Christian Ponder has a fractured rib and will not play Sunday against the Steelers.

Matt Cassel will replace Ponder in his Vikings debut.

"They just made a decision for what's best for me and my health and this team," Ponder said. "It stinks but it is what it is. But I'll be back for Carolina."

Coach Leslie Frazier said Ponder had an CT scan on Monday, which revealed the fracture. Ponder said the fractured rib is near his heart so doctors were concerned about him possibly doing more damage if he took a hit in that area.

Ponder suffered the injury early in the second quarter. He had a CT scan to make sure he didn't have a bruised lung.

"I guess their biggest fear was the location of it," Ponder said. "They're together right now, but if they were to separate, it's right in front of the heart, so that's their biggest fear."

Ponder said he's not thinking about the possibility of Cassel keeping the job if he plays well Sunday and leads the Vikings to a victory.

"I hope he plays well," Ponder said. "This team needs him to play well."

Cassel said he found out Friday morning that he's starting.

"I've been through a lot, but at the same time you get those butterflies," he said.

Cassel said he's not looking at anything beyond Sunday's game.

"I'm not really looking ahead to be completely honest," he said. "For me it's really about going out there and being accountable to my teammates."