Patriots players keep going down with injuries, but others keep stepping up.

“Guys really stepped up and made some big plays in the fourth quarter,” coach Bill Belichick said Sunday after the Pats, trailing the Rams by 16-13 in the final period in Foxboro, rallied to win, 23-16, upping their record to 5-2 and moving into a tie for first in the AFC East with Buffalo, which lost at Miami.

“I am really proud of them,” Belichick said. “We got contributions from everybody. There is no way you could go down the line and name them all, but a lot of guys stepped up, a lot of guys made big plays.”

But the question is — as the Patriots prepare to take on the struggling Colts this Sunday night in Indianapolis — how long can the banged-up Pats keep it up?

There were legitimate fears that New England’s playoff hopes went down with star quarterback Tom Brady, when he was lost for the season with torn knee ligaments in the first quarter of the first game.

After all, his replacement, Matt Cassel, hadn’t started a game since he was in high school.

But Cassel has stepped in and stepped up — never more effectively or dramatically than last Sunday, when he capped a game-winning, 53-yard touchdown drive with a perfectly thrown, 15-yard scoring pass to running back Kevin Faulk with 3:13 left to play.

The veteran Faulk is another who has stepped up, bigtime.

With the disappointing Laurence Maroney, New England’s No. 1 draft choice in 2006, already on the injured-reserve list and out for the season, and Sammy Morris and Lamont Jordan both sidelined by injuries, Faulk not only caught the game-winning pass against the Rams, but he also led the Patriots in rushing, carrying 13 times for 60 yards.

It’s bad enough that the Pats are playing without both Brady, the MVP of the NFL last season, when he set a record by throwing for 50 touchdowns while leading New England to a perfect 16-0 record, and Maroney, who led the team in rushing last year.

But the defense was dealt a blow Sunday, when veteran strong safety and inspirational leader Rodney Harrison went down for the year.

The secondary was already having problems, having lost All-Pro cornerback Asante Samuel to Philadelphia in free agency in the offseason.

The Chargers had burned the Pats several times on long passes in a 30-10 trouncing in San Diego, in which Philip Rivers threw for 306 yards and three TDs.

The Rams did likewise, albeit in defeat, as Marc Bulger threw for 301 yards, completing four passes of more than 20 yards — topped by a 69-yarder for a touchdown to rookie wide receiver Donnie Avery, who also had a 44-yard catch.

When cornerback Ellis Hobbs went to the sidelines with a shoulder injury in the fourth quarter, the Pats played down the stretch with a depleted secondary.

It included the likes of rookie Terrence Wheatley, Mike Richardson — a sixth-round pick last year who spent the season on injured reserve, and Antwain Spann, activated recently off the practice squad.

Now they’re heading for Indianapolis to face a future Hall of Fame QB in Peyton Manning.

This is not, however, the same caliber of Colts team the Patriots have played eight times in the previous five seasons — three of those in the playoffs.

Indeed, this Indianapolis team may not even make the playoffs, having lost four of its first seven games, leaving the Colts far behind undefeated Tennessee in the AFC South.

“This is definitely a different situation for us,” said Dallas Clark, Indy’s talented tight end. “But that’s the hole we’ve put ourselves in.”

Despite that, these Colts certainly are capable of putting some big numbers on the scoreboard against the injury-plagued Pats.

“I think Rodney’s a tough guy for them to replace,” Clark said of Harrison. “When you don’t have a guy like that, that’s tough.”

Especially when the Colts have receivers such as Clark, Marvin Harrison, Reggie Wayne and Anthony Gonzalez.

But not only does Belichick refuse to use injuries as an excuse, he refuses to even talk about them.

“We have had guys get hurt before,” he said this week. “That’s part of football. Every team has injuries. We just look at each game and try to figure out the best way we can to be competitive with the team we are playing. Every week it’s a challenge.”

With some of their best guys getting hurt almost every week, the challenge keeps getting tougher.

And so, with the Bills coming to Foxboro next week, on the heels of what promises to be a difficult game at Indianapolis this weekend, you have to wonder whether the determined but depleted Patriots can keep stepping up as players continue to go down.

jdonalds@projo.com