One question, though, remained.
How could the Patriots obtain only a second-round draft choice for a solid, young quarterback and a 12-year veteran leader, linebacker Mike Vrabel?
That mystery persisted Saturday after the Patriots announced the trade for the 34th pick in this year's draft, which the Chiefs earned with a franchise-worst 2-14 record.
Cassel, who hadn't started the previous seven seasons with Southern California and the Patriots, led New England to an 11-5 record, but no playoff berth, after Brady suffered a season-ending knee injury in the opener -- on a hit by Kansas City's Bernard Pollard, who said it was an "accident."
Cassel's steady improvement sent his stock soaring as he entered free agency. So the Patriots put the franchise tag on him, requiring any team that signed him to give them two first-round picks. But that tag would have cost the Patriots $14.65 million if they kept Cassel as insurance if Brady's health was questionable.
By shipping him out so early, they must be sure that 2007 NFL MVP Brady will be ready for the regular season after undergoing surgery for torn ligaments in his left knee on Oct. 6 and a later operation for an infection in the knee.
On Feb. 18, Brady said his recovery was on schedule for him to be ready for the opener.
"I'm feeling great. I'm feeling really good. Everything is progressing just as I expected," he said at a charity event.
Cassel and Vrabel join a team being run by new general manager Scott Pioli, who went to the Chiefs in January after serving as vice president of player personnel for the Patriots, the team he joined in 2002.
"I have a long history with both players," Pioli said. "Mike and Matt are men that I respect both personally and professionally. I look forward to having them as new members of the Chiefs family."
About a month after joining the Chiefs, Pioli fired coach Herm Edwards and replaced him with Arizona offensive coordinator Todd Haley.
Patriots coach Bill Belichick praised Cassel in a statement issued by the club.
"It is very easy to root for guys like Matt Cassel, who do everything the right way and flourish as a result," he said. "As much as we would have loved to continue working with Matt, we wish him nothing but the best as he takes this next step forward in his career."
The Chiefs needed help at quarterback after Brodie Croyle and Damon Huard were hurt and Tyler Thigpen, the first player from tiny Coastal Carolina to be drafted by the NFL, struggled as their replacement but did throw for 21 touchdowns and 2,994 yards.
The seventh-round draft choice in 2005 took over when Brady was hurt midway through the first quarter of the first game and went on to throw for 3,693 yards and 21 touchdowns and 11 interceptions while starting 15 games.
Vrabel fills a need for guidance and playing ability in a Kansas City defense that managed just 10 sacks all season and allowed a team-record 54 points to Buffalo. His departure also creates space under the Patriots' salary cap.
"Mike Vrabel epitomizes everything a coach could seek in a professional football player: toughness, intelligence, play, leadership, versatility and consistency at the highest level," Belichick said. "Behind the scenes, Mike's wit and personality is one of the things we have all enjoyed about coming to work every day."
But the Patriots restocked at linebacker last season with rookies Jerod Mayo, the Associated Press' defensive player of the year, Gary Guyton, Vince Redd and Shawn Crable. They also have veterans Tedy Bruschi and Adalius Thomas.
The day after their season ended, Vrabel lamented missing the playoffs.
"It's tough," he said. "You try to think that it's a pretty positive ending to a season, a good finish. You finish 11-5 and then you come in here and you realize that you're cleaning your locker out."
Cassel, standing nearby, looked to the future.
"This is a satisfying season for me," he said. "I proved a lot to myself and to a lot of others."