WALTHAM, Mass. — Here we are again. Series tied, 2-2. Game Five. TD Banknorth Garden.

Which Celtics team will show up tonight against the Pistons? The crew that came off consecutive road losses at Atlanta and Cleveland and won the pivotal Game Five in those series? The team that rebounded from its first home loss of the playoffs with a dominating performance in Game Three at Detroit?

Or the team that looked bad from start to finish Monday night in Game Four, shot 31.8 percent, scored 75 points and lost by 19?

Will it be the offensive Celtics triggered by Paul Pierce or the defensive Celtics anchored by Kevin Garnett?

“Who are we? I wake up saying I want to be both one night, and I think we will be,” coach Doc Rivers said with a smile yesterday after a review of the 94-75 Game Four loss.

Tonight’s Game Five against the Pistons would be the perfect time for the Celtics to play well at both ends of the court so they don’t have to return to Detroit for a must-win Game Six on Friday night. But predicting how a team will respond from one playoff game to the next is difficult, if not impossible. The Celtics won 66 regular-season games, the most in the league, yet they have been inconsistent throughout the playoffs.

Why? Teams become familiar with each other. There are no secrets in a seven-game series. There are no new X’s and O’s to draw. Adjustments are subtle. The mental game becomes paramount.

“As good as Game Three was (Celtics, 94-80), where I thought it was a perfect mix of great offense and great defense, it was the exact opposite in Game Four. That bothers you when you see that, and that’s how the playoffs have been,” Rivers said.

“There are changes. Each game is different. There’s a change in how they’re going to play, and that change can throw one guy off. If you throw one or two guys off on your team, it affects all five. They’re not trapping one game and trapping the next game, and you have one guy with the ball decide he wants to beat the trap instead of making the next pass,” he said.

And it’s more than trapping or not trapping. At this level, and at this time of year, everybody can play physically. Mental toughness determines who wins.

“After they won Game Two, they came back in Game Three and though they said they were ready, we didn’t see it,” Rivers said of the Pistons, the first playoff team to beat the Celtics on the parquet. “I felt that way last night. We wanted to win the game, but I didn’t think we acted like it because of the energy, the toughness.”

Indeed, the Pistons started fast, and the Celtics never matched their intensity.

“They’re going to bump, you bump ’em back. They’re going to foul, you foul ’em back. I just thought they had the upper hand. They were the first-strike team, and we were the retaliator. We were never the instigators. They instigated that entire game last night,” he said.

The Celtics’ coach never expected Game Four to play like that. He felt great before the game because his team was loose and had had a good shoot-around. The Celts seemed ready to play. But four minutes into the contest, Rivers realized it was going to be a long night.

“I saw it early on. They had a different sense of urgency than us,” he said. “When I saw that, it was let’s see if we can squeeze this game out somehow, keep it close enough to steal. And we did, but we just didn’t do it at the end. We had it to three. We had it to five. We had our chances. We really did. And as poorly as we played — and we played poorly — the fact that we had a chance is a good sign for us.”

Better signs are the Game Five and Game Seven victories over Atlanta and Cleveland. The Celtics were focused, locked in for those contests.

“They’ve been to two Game Fives and two Game Sevens, and they have responded,” Rivers said. “At this point it’s really about mental toughness and execution.”