FOXBOROUGH, Mass. — New England Patriots quarterback Tom Brady, the NFL's reigning MVP and one of the most consistent starters in league history, was scheduled for an MRI on Monday amid reports that he sustained a season-ending injury to his left knee.

Brady, whose streak of 128 straight starts is the third-longest ever, left Sunday's game against Kansas City after he was hit by Chiefs safety Bernard Pollard in the first quarter of New England's 17-10 victory. Patriots coach Bill Belichick offered no information on the injury, and Brady was not available for comment after the game, but reports circulated around the league that he had torn his anterior cruciate ligament.

"That's terrible. Terrible. I guess it's an ACL or something," Jets quarterback Brett Favre said Sunday. "They've always overcome injuries and things like that, but that's pretty difficult, Tom Brady."

Backup Matt Cassel said Monday he was ready to fill in for Brady next week against the New York Jets — and longer, if necessary.

"I'm not trying to be Tom Brady. I'm just trying to be Matt Cassel," he said when subbing for Brady on his regular weekly radio show. "I don't know where that's going to take us."

Cassel 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.

Cassel said on WEEI-AM that he hadn't talked to Brady, though the two had exchanged text messages. Cassel wasn't asked about Brady's injury.

Brady, 31, threw for a record 50 touchdowns and leading the Patriots through a perfect regular 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.

He missed the entire preseason with an unspecified foot injury, then left the season opener 7:33 into the first quarter when Pollard pulled him down by his left leg.

"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.

"They're looking at him, doing some tests on him, so I don't have any information there," the coach said, cracking a smile as he added, "doubt anybody's interested."

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 chinstrap and the guys rallied around me. I felt their support."

Cassel took over at his 2 yard-line and, after two handoffs nearly netted the Chiefs a safety, completed his first pass to Moss for a 51-yard gain. Cassel, Brady's backup for the past three years, finished the 98-yard drive with a 10-yard touchdown pass to Moss that gave New England a 7-0 lead.

"This is something I've been preparing for a long time," said Cassel, who threw just 33 passes while backing up Heisman Trophy winners Carson Palmer and Matt Leinart at Southern Cal. "It's not something I expected to come up on opening day."