11/20112: Boston, MA: The Spurs Danny Green (right) gets off a first half three point attempt over the Celtics Paul Pierce. The Boston Celtics hosted the San Antonio Spurs in a regular season NBA game at the TD Garden. section: sports topic: Spurs-Celtics (Globe Staff Photo/Jim Davis)
Paul Pierce closed out on the Spurs’ Danny Green Tuesday night during Boston’s loss.(Jim Davis / Globe Staff)
The Boston Globe

What’s most infuriating about the Celtics’ lack of defensive execution to those in the locker room, especially the head coach, is that the slippages are intermittent.

The Celtics aren’t a bad defensive team all the time. They can’t be accused of being unable to carry out the instructions of Doc Rivers and assistant coach Mike Longabardi, because they do — occasionally.

But the past week was deplorable, with opponents averaging 105.6 points per game in Boston’s three losses. On Wednesday night against the Spurs, the defense was almost nonexistent. Tony Parker toyed with the Celtics as if it were a scrimmage against Alamo Community College.

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The Celtics allowed 112 points, which was not a season high (they yielded 120 in the opener to Miami) and the Spurs flirted with shooting 60 percent for the game; they did convert 60.5 percent of their shots in the second half. San Antonio battered the Celtics on backdoor plays, with reserve Tiago Splitter attempting most of his 11 shots within 2 feet. He had nine field goals.

It was a discouraging display, one that can’t be overlooked with the Oklahoma City Thunder — who are younger and more athletic than the Spurs — at TD Garden Friday.

With Thanksgiving off, the Celtics could only reflect on their mistakes, not work on them. They will have a shootaround Friday to sharpen some things before facing Kevin Durant and Russell Westbrook.

But the numbers show that the Celtics can play defense — when they want to.

In their six wins, the Celtics are holding opponents to 42.1 percent shooting and 91.5 points. In their six losses, those numbers balloon to 50.4 and 107. That’s what has Rivers so angry, the fact that the Celtics are showing up to play defense when they have the energy and passion. But every other night, they don’t.

“It’s difficult, very difficult,” said Rivers. “I don’t know if there’s a word more than that. But if there is, then it’s that. I’m concerned with the way we’re giving up points. Listen, there’s a lot of good things I see, but defense is not one of them.

“It’s shocking in the games we win how good our defense is, and in games we lose, how bad it is.”

It’s not that Rivers’s defensive concepts have suddenly become ineffective, but the execution has been spotty. Rajon Rondo has constantly allowed opposing guards to dive into the lane, and there was no better example than Wednesday, when Parker danced like Fred Astaire into the key, and when the Celtics’ bigs were sucked in, he simply dished the ball to Splitter or another big man who then found Splitter.

It was simple, but the Celtics were helpless to stop it.

“It comes from everybody,” said Paul Pierce. “Not only our bigs. Our guards have to sink in on the penetration a lot of times. We’re too concerned about shooters out there and rightfully so. But I think we have to do a better job with our help defense. We’ve got to be an in and out team, where we help the penetrating drives and also get out to our shooters.

“We’re a grind-it-out team. We don’t mind playing games in the 80s and 90s where we’re getting defensive stop after defensive stop. We have to go back to that and understand who we are, because the last few games, that’s not who we are.

“For us to do damage in the Eastern Conference, that’s that type of team we’ve got to be. It’s not just one particular season. We’ve got to take it personal as a team and go out there and do something about it.”

The breakdowns are team-wide. It starts with Rondo, but it also entails the Celtics’ refusal to get back on defense. When Rondo penetrates to score and is slower to get back downcourt, a teammate needs to cover the opposing point guard. Parker spent Wednesday leaking out after Rondo drive attempts and jump-starting the Spurs’ break.

“It’s everybody really being on the same page,” Pierce said. “When you have one breakdown on the defensive end, it breaks down all five guys. It’s like a domino effect where you have another man help and another man help and then it leaves wide-open shooters.

“It has to come from all of us. Those are things you have to do to be a great defensive team.”

Rondo holds himself responsible for not stopping dribble penetrations, and it’s becoming more of an issue, especially against some of the game’s elite guards. Friday night, Rondo gets another difficult assignment in Westbrook.

“We’re making the same mistakes,” he said. “We’ve just got to do a better job of focusing at shootarounds in the mornings when we’re given an assignment.

“It’s a collective team effort. It starts with me. I’ve got to do a better job of pick-and-roll coverage and help the bigs rebound.

“Each team varies, so we’re making a lot of easy mistakes that allow teams to get behind our defense to score points. We pride ourselves on defense and we’re just not getting it done.”