Vikings quarterbacks Matt Cassel, left, and Christian Ponder stretch during their football practice at the Grove Hotel in Watford, England, Thursday Sept. The Pittsburgh Steelers are to play the Minnesota Vikings in the NFL International Series at Wembley Stadium in London on Sunday, Sept 29. AP Photo/Sean Ryan, NFL Viking quarterbacks Matt Cassel, left, Christian Ponder, center, and McLeod Bethel-Thompson in action during their football practice at the Grove Hotel in Watford, England, Thursday Sept. LONDON (AP) - Matt Cassel will start at quarterback for the Minnesota Vikings in place of injured Christian Ponder against the Pittsburgh Steelers at Wembley Stadium on Sunday. The Vikings said Friday that Cassel will replace Ponder, who sustained a fractured rib in last weekend's 31-27 loss to the Cleveland Browns. "We just felt like it was, from a medical standpoint, too risky to put him out there to take a hit," coach Leslie Frazier said Friday at the team's practice facility. He's a tough guy but it probably wouldn't be wise to put him out in that situation. Cassel hasn't played a down since last season at Kansas City, where he threw for six touchdowns and 12 interceptions in nine games for the 2-14 Chiefs. "I've been preparing for this and I feel like I'm ready to go," Cassel said Friday. I feel comfortable with this offense. I'm excited about it. That's what you prepare for as a backup. Frazier said the Vikings made the decision with a bye week coming up to give Ponder more time to recuperate. He didn't foresee any quarterback controversy. "We worked Matt with the mindset that this was a possibility," Frazier said. Ponder, a third-year quarterback, has thrown five interceptions and two touchdowns for a 65.9 passing rating this season. He also has lost two fumbles. The Vikings and Steelers are both 0-3 heading into the NFL International Series match at Wembley Stadium.