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FOXBORO - So begins another round of the peculiar institution known as the NFL Preseason.

The defending AFC-champion New England Patriots play tonight for the first time since Feb. 3 in Glendale, Ariz., taking on the new-look Baltimore Ravens at Gillette Stadium (7:30 p.m.; Ch. 5, 64) in a game that costs full fare but delivers less than a third of the satisfaction to fans who want to see their heroes in action.

They won't see many of the people who started Super Bowl XLII, that's for sure. There may be token appearances by some of the starters for a series or two, and it's not certain that Tom Brady, the Patriots' globe-trotting quarterback, will even be in uniform.

Brady will have to play some in this preseason because even future Hall of Fame quarterbacks need to blow off the rust under game conditions. But there are at least two other games (a week from Sunday in Tampa, and Aug. 22 at home against the Eagles) in which Brady can get his preseason reps.

Right now, as the NFL preseason has evolved over the last decade or so, the first game is for evaluating players who are new to the system and need to prove that they can execute at full speed what they've learned during workouts at three-quarter-speed or less in Foxboro over the past two weeks. As Patriots' coach Bill Belichick said earlier in the week, there is no set formula for how this evaluation is to be conducted, and no preseason "Bible" that dictates how playing time should be apportioned.

"It's hard for me to speak on what anybody else is doing," Belichick said. "I think what we try to do, and what my philosophy is on preseason and training camp is, is that we have two goals. One is to get our team ready for opening day, and the other is to get our team ready for a 16-game regular-season schedule. Whatever we feel like is best to do then that is what we try to do.

"That takes into account a lot of things," he continued. "It takes into account every single player; it takes into account the different units and sometimes the units within the unit. Whether it is the offensive line with the offense or the secondary with the defense or maybe it is the specialists in the kicking game - whatever it happens to be. I don't know if there is necessarily a right formula. We don't have some type of blueprint where we can say, 'OK, this is what we're going to do.'"

A good example may be how much time will be given the offensive linemen tonight. The numbers are down with guard Stephen Neal's continued presence on the physically-unable-to-perform list and tackle Matt Light's extended absence from recent practices for undisclosed reasons. As a result, those who have who have been taking the bulk of the snaps in camp may get extended play because there just isn't room for much substitution.

Belichick has always been one to determine playing time according to actual need as opposed to the football equivalent of a "pitch count." If he and his staff determine beforehand that either an individual player or a position group should get more snaps in order for them to perform a valid evaluation, so be it.

"We always talk about it before each season and each game," he said. "Sometimes the needs of the team vary or the needs of a particular player vary from one year to the next, one game to the next or one week to the next. That's how we try to do it."

Over the course of the first three preseason games, a pattern emerges. Gradually the focus shifts from evaluation to execution, to the point where the third preseason game is usually considered a dress rehearsal for the regular season.

Because the preseason has been condensed into a four-game schedule since 1978, the process of switching from evaluation to execution seems faster, Belichick said.

"I do agree there is kind of a build-up," he said, "but I think it's always been that way. Even if you go back to when there were six preseason games, that was kind of the way it was. Guys play a quarter, a half, three quarters then a full game in the fourth and fifth game. Then they play a little less in the sixth game. I don't know that it has changed. I think it has always had an element of that."

However, it's not uncommon these days to see teams hold out most if not all of their starters in the final preseason game to prevent the sort of disaster that befell the Patriots on Sept. 1, 1989, at then-Sullivan Stadium. In the final preseason game of that year, the Patriots lost three defensive starters - linebacker Andre Tippett, cornerback Ronnie Lippett and defensive end Garin Veris - to season-ending injuries in a 16-0 loss to the Green Bay Packers. With such devastating losses to the defensive lineup, the '89 Patriots surrendered 16 or fewer points to an opponent only three times and finished 5-11 - the first time they had been under .500 in eight seasons, which cost head coach Raymond Berry his job.

Now, the fourth preseason game is almost a "bonus" opportunity for players on the periphery to prove they belong on an NFL roster. And, new rules about roster reduction make it more advantageous for coaches to play the "bubble" athletes more in the final game because they have more of them in camp.

There is only one mandatory preseason cut before the final 53-man roster has to be set. Two days before the Patriots finish their preseason schedule against the Super Bowl-champion New York Giants (on Aug. 28), they must reduce their roster from 80 players to 75 - which should give Belichick plenty of options for keeping his first-stringers safe from harm.

"With the roster rules changing, that changes the way the games are played," Belichick said. "In the past, we went with whatever number we started with - to 60, to 55 and then to 45. This year, the last cut is 75. You go from 80 to 75 to 53. If you only had 55 players, then your pool of players is a lot smaller then when you had 75."

No matter what, Belichick expects all of his uniformed players to be ready to play in the preseason, even if he has no intention of using them.

"Things can change, and usually do," he said. "Each player has been told to be ready to go. When the decision comes for him to play, then that is a decision we make, not them. They need to be able to make the most out of it."

MARK FARINELLA may be reached at 508-236-0315 or via e-mail at mfarinel@thesunchronicle.com. Read Farinella's blog, "Blogging Fearlessly," at thesunchronicle.com/farinella.