Phoenix ? An already difficult second NFL season came to an end for Arizona’s Matt Leinart on Tuesday when he was placed on injured reserve with a broken collarbone.

Kurt Warner, the 36-year-old quarterback who had shared duties with Leinart, moves into the starting job. The team signed Tim Rattay on Tuesday to be Warner’s backup.

Leinart, a left-hander, fractured his left collarbone when he was sacked by Will Witherspoon in the second quarter of the Cardinals’ 34-31 victory over the Rams in St. Louis on Sunday. He sat on the sideline in the second half with his arm in a sling.

“We didn’t want to rush him back,” first-year coach Ken Whisenhunt said. “To hold a roster spot for that long is difficult, with some of the areas we are banged, with the hope he can get back in time. I am more concerned about him trying to rush back and maybe jeopardize his future.”

The uncertain timetable was a major reason for calling an end to the young quarterback’s season.

“Whenever you have a fracture there will be six weeks or however long it takes to heal,” Whisenhunt said, “and then, especially because it is his throwing shoulder, you have to do the rehabilitation of it throwing the football. Who knows what it could have been? Could have been 8 weeks, could have been 12 weeks.”

Leinart, the 10th overall pick in the 2006 draft, had been unhappy with the two-quarterback system employed by Whisenhunt. The former Heisman Trophy winner and two-time national champion at USC has started 16 games for Arizona, including the first five this season.