If there was one thing that really bothered the Celtics in their Game 2 victory over the Knicks it was their lack of defensive rebounding. Even without Amar’e Stoudemire for most of the game, New York still managed to grab 20 offensive boards, which allowed the Knicks to stay in the game despite shooting 35 percent.

This was first explained away after the game by noting that the Celtics had to run double-teams at Carmelo Anthony, but that didn’t start until late in the fourth quarter. Something else was going on.

Celtics coach Doc Rivers put some of it on dribble penetration, but the bigger problem has been their help defense. “We’re helping but we’re not trapping and when the shot’s go up we’re just turning and going to the glass,” Rivers said. “That means the guy who was in help, there’s no one on his guy’s body. We showed that. That was our first 17 [film] clips.”

The Knicks have also clearly made attacking the glass a priority. “They’re sending all five guys to the glass and they’re not even trying to get some rebounds, they’re just trying to keep the ball alive as long as possible,” Rivers said. “They feel they have a speed advantage.”

Anyone who has followed this team over the last few seasons knows that defensive rebounding has been an issue. The Celtics survived last season until Game 7 against the Lakers when the problem became insurmountable. They have improved this season, ranking a respectable ninth in defensive rebounding percentage, which is mostly due to the efforts of Kevin Garnett who re-emerged as a dominant defensive rebounder.

But Garnett needs help. Jermaine O’Neal has just two defensive boards through the first two games, while Glen Davis had has a problem keeping the taller Jared Jeffries (nine offensive boards) off the glass.

Defensive rebounding is vital for the Celtics because that triggers their transition game and allows Rajon Rondo to create havoc in the open court. No boards, no running game and the Celtics halfcourt offense has not clicked yet.

This is their biggest item to try and fix for Game 3 because getting stops and controlling the glass is the key to everything they want to do offensively, but it’s not the only one. Here are three more:

CAN THE BENCH PRODUCE

Here’s how bad the play of the Celtics reserves has been: The much-maligned Jeff Green is shooting the highest percentage of any of the four bench players at 33 percent. Green, Davis, Delonte West and Nenad Krstic are a combined 9-for-30 from the floor and the problems only start there.

Rivers, as always, believes that the second unit’s offensive problems stem from the defensive end. They’re not getting stops and thus are forced to walk the ball up the court and they’re struggling to get good shots. The result has been blown leads and two down to the wire games that frankly could have gone either way. “Whatever it is, we’ve got to figure that out,” Rivers said.

Green is taking the brunt of the criticism and he has been nearly invisible, except for a couple of brief flurries in both games. The invisible production part is not helping the perception that he’s somehow overwhelmed by the stage, but Green is simply a low-key personality.

“He’s a little more aggressive on the court,” Davis said. “He’s nice in real life. That’s just the way Jeff is. Jeff’s a great guy. His game is different than his personality.”

Nice guy or not, Green has to start being more assertive offensively when he’s on the court. He’s the one player among the reserves who can create his own shot, either off the dribble or post-ups and they desperately needed some kind of offensive spark.

“Jeff Green, number one, he has a poker face so that’s misread,” Rivers said. “It’s like he’s not interested. He is interested. Number two, he’s playing extremely hard he’s just not getting a lot done right now. The third thing is he’s got to himself in the right places in our execution that will allow him to be more successful on both ends.”

He’s also not the only one who has struggled. Krstic has been largely irrelevant in this series playing just eight minutes in the two games and none in the second half of either. He’s had bone bruises on both knees but he insists that he’s healthy.

“He’s fine and he can play more minutes,” Rivers said. “He knows that. He’s hurting but so is everyone. If they’re hurt enough they’ll tell me.”

If Rivers really wants to shake things up he could try using Toy Murphy in Krstic’s spot. In very limited minutes, Murphy still showed the ability to rebound and while his shot was clearly off, he did drop nine points in the regular season finale against many of the same reserves the Knicks have used in the playoffs.

But the core of the reserve crew will remain the same and they simply must start playing better to give the big four a break. Things are different in the playoffs. Rotations are shortened, there are no back-to-backs and there’s more time for rest and preparation. All that is true, but it doesn’t change the fact that Rondo, Paul Pierce and Ray Allen have each played more than 80 of a possible 96 minutes in the first two games.

They’ve been needed. Rivers has decided that he wants to match Pierce on Carmelo Anthony whenever he’s on the floor and the Celtics offense has been virtually nonexistent without Rondo. Still, they should be fresh. The two-day break came at a good time and the Celtics went 15-5 this season when they had more than one day off between games.

JARED JEFFRIES HAS BECOME AN X-FACTOR

If you only watched the highlights you saw the Knicks forward get scored on in the post by Kevin Garnett and then get stripped by Garnett on the other end of the court in the final 19 seconds of the game. But Jeffries has been a problem for the Celtics in this series because of his ability to get on the offensive glass (again, nine offensive boards already) and his defense on Rondo.

The big guy defense has been used by other teams in the past against Rondo and Jeffries has been effective at deterring Rondo’s drives to the basket as well as playing off him to provide strong help defense.

Rondo can expect to see a lot more of the 6-foot-9 Jeffries because Chauncey Billups is battling of a strained tendon in his left leg and Toney Douglas was turned inside out in Game 2. Using Jeffries on Rondo should open the door for mismatches, but the Celtics lack a post presence beyond Garnett and while the Knicks aren’t big, they are deep in tough, athletic swingmen like Bill Walker, Landry Fields and Shawne Williams.

Beyond the individual brilliance of Anthony and Amar’e Stoudemire, that’s what the Knicks a tough matchup. They are essentially position-less and for all the criticism he’s taken, Knicks coach Mike D’Antoni has used unorthodox combinations to great effect while trying to cover for the absence of some of his key players.

WALKING INTO A BUZZSAW

The Celtics have long been proud of their ability walk into a hostile arena and cut another team’s heart out. They did it in Cleveland and Orlando last season, but neither will be anything like the scene at Madison Square Garden on Friday.

“I think our guys love that atmosphere,” Rivers said. “Road teams, you always like that. I think it’s fun to go on the road.”

It will be fun. It also won’t be easy. The Celtics have lived dangerously in the first two games of this series and if they give the Knicks life, this series could take on epic proportions. Going back to Boston with anything less than a 3-1 lead will be asking for trouble.

“We have to do a lot better,” Rondo said. “We have to start games better, close quarters better. It’s playoff basketball. We have two wins but it doesn’t start until somebody gets a win on the road so hopefully we can get the first win in New York.”