DALLAS -- Mavericks owner Mark Cuban made a point to publicly challenge superstar Dirk Nowitzki to play with more effort and energy defensively.

"I think we've got to get a little bit more out of Dirk defensively," Cuban said before Monday night's game against the Boston Celtics, responding to a general question about the Mavs' recent performance. "I think [the Mavs need] a little bit more energy out of Dirk, particularly when things don't start well."

It's rare for Cuban to publicly criticize Nowitzki, a 12-time All-Star who is the face of the franchise and was the 2011 Finals MVP. Cuban kidded that they have a rule that allows him to call out Nowitzki once every 10 years.

The timing of this criticism is odd, considering the Mavs had won four of their last five games, including a rout on the road against the Oklahoma City Thunder on Sunday night. But Cuban, who watched the games on television while vacationing with his family on spring break, wasn't satisfied with the lack of effort he saw from Nowitzki on defense.

Cuban said he mentioned his concerns to Nowitzki during a brief, sarcastic conversation Monday, asking the 16-year veteran how his nap was while he was on the floor against the Thunder.

"Dirk always goes through a little slump during the year when he needs to re-motivate," Cuban said. "I think we've seen that for a couple of games, and I think he's going to come back stronger, particularly after three days [off] and those naps he's gotten in the last couple of games other than Utah."

Nowitzki, the Mavs' leading scorer with 21.4 points per game, took the criticism in stride when informed of Cuban's comments after Dallas' 94-89 win over the Celtics.

"I've got to compete and help the guys set the tone," Nowitzki said. "Some nights, that's defensively or on the rebounding. Some nights, it might be more scoring. I usually take pride in being a good all-around player. Especially in this homestand, if we want to make the playoffs, we've all got to do a little more."

Nowitzki's poor defense was heavily scrutinized earlier in his career, but he has developed into a solid, savvy team defender. His defensive rating (105.0 points allowed per possession) is the second best among the Mavs' rotation players, ranking behind only center Samuel Dalembert and reserve wing Jae Crowder.

Over the last five games, Nowitzki has a defensive rating of 98.3, although that isn't necessarily an indication of effort. He's had some offensive struggles recently, going 7-of-25 from the floor in a two-game span against the Indiana Pacers and Golden State Warriors. After the loss to the Warriors, Nowitzki vowed to be more aggressive, especially early in the game, and responded by scoring 31 points on 12-of-14 shooting in a win over the Utah Jazz the next night.

"It's not about getting involved early, because you don't need to be involved offensively to be there defensively," Cuban said. "You know when Dirk's trying to get 10 rebounds and you know when he's not. And we need a little bit more of those try-to-get-10 games, because I think our starters in particular respond to his energy."