"I think it was pretty obvious we've got a lot of work to do, based on tonight, in all three phases of the game," said Belichick, who played down the significance of Brady not playing.
Brady remained at home, reportedly to rest a sore foot, and Cassel had limited success moving the ball against a feisty Bucs defense that scored on his third-quarter fumble and intercepted the first pass thrown by successor, Kevin O'Connell.
Asked if Brady would have played had it been a regular-season game, Belichick said: "Well, it's not a regular-season game."
And, would he characterize Brady's not playing as precautionary? "I wouldn't characterize it," he replied.
Bucs starting quarterback Jeff Garcia also has yet to play this preseason after missing most of training camp with a sprained right calf. His absence is creating plenty of opportunities for Brian Griese, Luke McCown and Chris Simms to build their cases for being No. 2.
Griese and McCown produced a 17-3 halftime lead, with the Patriots scoring on Stephen Gostkowski's 53-yard field goal on the final play of the second quarter.
Blitzing safety Sabby Piscitelli scooped up an errant shotgun snap to Cassel and ran 24 yards for a touchdown that made 24-3 early in the third.
Griese, listed third on the depth chart, got the start and directed an impressive 17-play, 80-yard TD drive on the first possession of the night. Earnest Graham finished the nine-plus minute march with a 1-yard run.
"It was a good opportunity for us to go against a quality defense," said Griese, who worked two series and completed nine of 10 passes for 44 yards. "I know they had some guys out, but this team is a really good team. And for us to come out and put a drive together to start the game was good for our confidence."
McCown, who entered camp No. 2 after winning one of three starts last season, led a drive that produced a 34-yard field goal and threw a 1-yard TD pass to Byron Storer to complete an eight-play, 86-yard drive in the second quarter.
The sixth-year pro, a former 1,000-yard receiver who hasn't played in the NFL since 2006, had two receptions for 37 yards and also gained 16 yards on a reverse during the Bucs' game-opening drive.
"I played with Luke before, so it was an opportunity for us to run some old plays," Bryant said of his 33-yard catch and run.
"He and I played together in Cleveland, so we have a way of connecting. I kind of had an idea he was going to throw it because of the way the coverage looked. We just happened to make the play."
Randy Moss, the other half of New England's record-breaking passing combination, made his preseason debut and caught two passes for 25 yards. Both receptions came on the five-play drive Cassel used to get the Patriots on the scoreboard at the end of the first half.
Safety John Lynch, who helped the Bucs win the Super Bowl six seasons ago, made his New England debut, despite only joining the team on Friday.
Lynch, entering his 16th season, was announced as a starter, giving the crowd of 63,365 a chance to give him a nice ovation. It was only ceremonial, though, as Brandon Meriweather and Antwain Spann were on the field as planned for the first snap.
Meriweather left in the first half with a leg injury and did not return.
Cassel was 6-of-10 for 57 yards before being replaced by O'Connell, who led a fourth-quarter touchdown drive that Heath Evans finished with a 1-yard run.
"We got some experience against a good team, against a fast team, against a playoff team," Belichick said. "There's a lot of things we're going to need to do better."