FOXBORO -- Being ready to play is not the same as expecting to play.

Matt Cassel is always prepared to go into a game. But he seldom gets the chance.

Until yesterday.

"It's the first day, opening day; you don't know if you're going to play, or if you're going to play three downs in the fourth quarter, like I did last year," he said.

When Tom Brady went down with a knee injury midway through the first quarter, Cassel was thrown into the huddle on the Patriots' next possession.

Normally a mop-up man, he suddenly was The Man.

"I've been here four years," Cassel said, "and every time Tom's been knocked down, he's gotten right back up. I was anticipating that being the next move. But they said, 'You're going," so I just buckled my chinstrap and got ready to go."

There were some pundits who predicted Cassel would be gone on Aug. 31, when the Patriots had to trim their roster to 53 players.

Although he started all four preseason games, he hadn't thrown a touchdown pass, or even directed a TD drive. That led to speculation that, even though he'd been on the team since 2005, when he was drafted in the seventh round out of Southern Cal, he'd be waived in favor of Matt Gutierrez, a third-stringer last year, and this year's third-round draft choice, Kevin O'Connell.

It didn't help that Cassel hadn't started a game since high school, in Chatsworth, Cal. In college, at Southern Cal, he played behind two Heisman Trophy winners: Carson Palmer, now with the Bengals, and Matt Leinart, now with the Cardinals. But he's always prepared in the event he had to go into a game on a moment's notice.

"I've been waiting for this opportunity for a long time," he said.

Brady's opportunity came in the second round of the 2001 season. A sixth-round draft choice out of Michigan the year before, he came off the bench when Drew Bledsoe was injured against the Jets and has started every game since, leading the Patriots to four Super Bowls and three NFL championships.

Cassel couldn't have entered yesterday's game in more challenging circumstances.

His first drive began at his own 2-yard line and, two running plays later, the Pats were backed up inside the 1. That's when, on third down, he lofted a long pass to Randy Moss for a momentum-creating 51-yard gain.

"It was a play-action pass," Cassel said. "Randy made a great move to get on top of the guy, and I just tried to put it in the vicinity. He did a great job coming down with the ball.

"It was nice to get out of the end zone and get going -- get that first pass out of the way and to have a nice completion like that."

That completion was the big play in what turned out to be a 98-yard touchdown drive, culminating in another pass from Cassel to Moss, this one a 10-yarder at the back of the end zone, almost under the left upright.

For the day, Cassel completed 13 of 18 attempts for 152 yards and that touchdown, without throwing an interception. He was sacked twice.

"I got a lot of reps during the preseason," he said, "and that ended up being pretty fortunate for me. I prepared the same this week as I did in the preseason, and went out there and took the same approach."

Expect it's not quite the same in the regular season as it is in the preseason.

"There's an adrenaline pump that went on immediately," Cassel said. "You just have to take a deep breath and get going. You have to focus on the job at hand. That pass (to Moss, out of the shadow of the goalposts) definitely calmed the nerves down a little bit. I said: 'I can do this.' "

It helped that his teammates have confidence in him.

"He knows what to do back there," said Matt Light, the Pats' Pro Bowl left tackle.

"He was very calm, cool and collected," wide receiver Wes Welker said. "Especially on that first drive. That first throw, when we were backed up like that, was huge."

"That showed his arm strength," veteran linebacker Tedy Bruschi said, "and got us fired up."

Cassel is fired up about the prospect of starting next Sunday against the Jets at the Meadowlands.

"You never wish injury on anybody," he said. "I don't know what Tom's circumstances are, but I'm going to approach it like I have each and every week. I'm going to prepare the same way. I'm going to go in with the same energy and excitement and try to get ready for the Jets.

"You never know when you're going to play. This is a great example of that."