Ponder was injured on a hit in the second quarter last Sunday against the Cleveland Browns, and he told the team's medical staff Monday he was feeling pain. Initially concerned he had a bruised lung and wouldn't be able to fly to England for Sunday's game at London's Wembley Stadium, trainers put Ponder through a CT scan that revealed the fracture.
Because the broken bone is so close to his heart, Ponder said, the Vikings (0-3) made the decision to hold him out against Pittsburgh (0-3) to avoid risking a more serious injury.
"We just made a decision, what's best for me and my health and for this team," Ponder said Friday. "It stinks, but it is what it is, and I'll be back in for Carolina [on Oct. 13]."
Ponder will get a chance to heal during the Vikings' bye week. The former first-round pick has thrown five interceptions to just two touchdown passes in three games, having an interception returned for a touchdown in a 31-30 loss to the Chicago Bears in Week 2.
The Vikings signed Cassel to a two-year, $7.4 million contract in March after he was released by the Kansas City Chiefs. Coach Leslie Frazier and general manager Rick Spielman said at the time that Ponder was the starting quarterback and Cassel was coming in to give the Vikings a veteran option in case something happened to Ponder.
Minnesota was forced to start Joe Webb after an elbow injury kept Ponder out of a wild-card playoff game at Green Bay in January. Webb completed just 11 of 30 passes for 180 yards in a 24-10 loss.
Cassel, 31, emerged in the NFL after Tom Brady tore his ACL during the first week of the 2008 season. Cassel led the New England Patriots to an 11-5 record (they did not make the playoffs). He reached the Pro Bowl and led the Chiefs to the postseason in 2010, but injuries and ineffectiveness made him expendable after Kansas City traded for Alex Smith last winter.
"I've been through it a lot, but at the same time, you still get those butterflies and everything," said Cassel, who backed up Carson Palmer and Matt Leinart while at USC. "I just found out this morning, right before practice. But at the same time, we've been preparing for this. I had a good week of practice. I feel like I'm ready to go."
The rest of the players on the Vikings' injury report all practiced in full Friday and are probable for Sunday's game. They are running back Matt Asiata (hamstring), cornerback A.J. Jefferson (ankle), defensive tackle Kevin Williams (knee), linebacker Erin Henderson (heel) and long snapper Cullen Loeffler (neck).
Information from The Associated Press was used in this report.