"He was in a lot of pain," he said. "When you hear a scream, you know that."
Various reports Sunday night said Belichick told the players that the injury was serious. The Patriots were already looking for a possible replacement for Brady. A person familiar with the situation told The Associated Press that former Tampa Bay quarterback Chris Simms was coming in Monday for a physical.
Brady was the NFL MVP last season after throwing 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.
Any chance at redemption might have disappeared when Brady, who has started 128 consecutive games, went to the turf clutching his left knee midway through the first quarter. After being tended to on the field, he walked off, limping, between two trainers.
Brady, 31, went to the locker room and wasn't seen again on the sideline as backup Matt Cassel led the Patriots to a 17-10 victory. The two-time Super Bowl MVP was not available for comment after the game, and Belichick said he had nothing to add to the in-game announcement that it was a knee injury.
"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."
The Chiefs also lost their starting quarterback when Brodie Croyle was sacked in the third quarter. The team said he had a bruised right shoulder and he was replaced by Damon Huard.
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 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."
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.
In the process, Brady has become one of the league's biggest stars and a crossover cover boy who has met the Pope and the president, dated actresses and supermodels and rewrote one of the NFL's most coveted records.
Last year, while leading New England to a 16-0 regular season, Brady set a record with 50 touchdown passes and improved his overall record as a starter to 100-27 - the best in the Super Bowl era. The Patriots blew a chance at an unprecedented 19-0 season and a fourth NFL title with a 17-14 loss in February's Super Bowl to the New York Giants.
Brady did not play in four exhibition games this summer - all of them Patriots losses - while trying to recover from a right foot injury.
"He's the face of the New England Patriots, and Tom being who he is it kind of hurts, to be honest with you," said Moss, who caught 23 of Brady's TD passes in 2007, also a record. "I know the show must go on. Hopefully Matt Cassel is ready to step in. I know the team is ready to embrace him and let him lead us."