Cassel, who even was a backup at Southern California - to Heisman Trophy winners Carson Palmer and Matt Leinart - could have some competition. A person familiar with the situation told The Associated Press that former Tampa Bay quarterback Chris Simms was heading to Foxborough on Monday for a physical, and reports elsewhere said Tim Rattay was also being considered.
Belichick, who runs one of the most secretive operations in the league, denied the team reached out to any other quarterbacks.
"In spite of what some people are putting out there, we haven't worked out anybody," he said. "We had a lot of people call us, I can tell you that."
For now, Cassel is his quarterback.
"The opportunities that Matt got, experience always helps," Belichick said. "I have a lot of confidence in Matt."
Belichick stopped short of calling the hit by Pollard dirty. He did emphasize his players are always taught it's their responsibility to hit the QB above the knees and below the shoulders.
The league said the play was reviewed by Mike Pereira, the supervisor of officials, who determined it was legal.
"It is not a foul because the defensive player was coming off and affected by a block by the offense," NFL spokesman Greg Aiello said.
Brady, 31, threw for a record 50 touchdowns in leading the Patriots through a perfect 2007 season to the brink of their fourth Super Bowl title in eight years. But New England lost its chance at an unprecedented 19-0 record when the Giants won the championship, 17-14.
A former fourth-stringer who was the 199th overall selection in the 2000 draft, Brady himself took over at quarterback when longtime starter Drew Bledsoe sustained a life-threatening chest injury in a 2001 game against the New York Jets. Brady led the Patriots to their first NFL title that year, another in 2003 and another in 2004.
But he missed the entire preseason with an unspecified foot injury, then left the season opener 7:33 into the first quarter.
"He was in a lot of pain," Pollard said. "When you hear a scream, you know that."
After being tended to on the field, Brady walked off, limping, between two trainers. He went to the locker room and wasn't seen on the sideline again.
Brady was 7-for-11 for 76 yards, completing a 28-yard pass to Randy Moss on the play in which he was injured. Moss fumbled the ball away when he was tackled; the Patriots forced Kansas City to punt, then Cassel came in for New England.
"Since I've been here and been around Tom, he's always popped back up," Cassel said. "I didn't know (how serious it was). I just buckled my chin strap and the guys rallied around me. I felt their support.
"This is something I've been preparing for a long time. It's not something I expected to come up on opening day."