As travel companions go, giving up a season-high 121 points to Cleveland on Sunday and wiping that off the top of the board by allowing the Clippers 129 on Wednesday are the equivalent of a child kicking at the back of your seat on a long flight home.

The Celtics departed for the All-Star break yesterday and won’t meet as a team again until next Wednesday’s practice. Their last moments together have not been pleasant, a pair of double-figure losses featuring defensive lapses and an embarrassing lack of grit.

Among the factors easiest to pinpoint for the slide is the absence of Marcus Smart, whose anger would be helpful now but most certainly wasn’t when he punched a picture in his hotel room on Jan. 24 in Beverly Hills and lacerated his right hand on the frame.

We caught up with Smart late Wednesday night, and he has progressed to the point where all he’s beating up now is himself. Smart knows his actions have deprived his team of an important and badly needed element, and he admitted he hasn’t let the Marcus Smart from the hotel room off the hook.

“I’ve been on myself pretty bad,” he said, “especially when I have to just sit back there and watch those guys struggle, and there’s nothing I can do. But we’ve got the break coming up. We’ve got to come back and be ready.”

In eight of the 11 games he missed, the Celtics have given up more points than their season average. They’ve surrendered 105.6 per outing in that stretch, raising their overall allowance to 99.6 a night.

To be fair, however, the problems were coming to the surface prior to that. In the five games before Smart lost out to the picture frame, the C’s gave up an average of 103.8, and that’s even with holding Philadelphia to 89 in a nine-point loss in the Garden on Jan. 18.

Brad Stevens is loath to utter anything resembling an excuse, but after Tuesday’s practice he noted that Smart and Shane Larkin (knee) were needed against Toronto.

Yet the coach could have said the same for all the games they’d missed.

The fact Smart’s own decision-making led to his place on the inactive list only serves to compound the problem as the Celtics go through perhaps their worst run of play. They lost 5-of-6 last month, too, but those games were more competitive than three of the recent four defeats.

“Man, it’s been real tough just as a competitor,” Smart said. “And I have to sit there and watch it, and there’s nothing I can do to help my team. It’s tough, but regardless of that, we’ve still got to figure out a way to fix it and win games. In the NBA, guys are going to get hurt and guys are going to be missing games, and you’ve just got to find a way to win.”

Asked if his not being available is the reason for such bad defense the last two games and beyond, Smart said, “I don’t know. We’ll see. But, you know, I don’t think so. We’re a really good team. We’ve just had a couple of mental lapses. These guys are great defenders, and once they put their mind to it, they can play. It just wasn’t there.”

As for the reason the C’s have fallen off of late, Smart was blunt.

“Effort,” he said. “Other teams want it more than us right now. I think we got a little complacent, and we’ve just got to realize, especially going into the last part of the season, that teams are gunning for us. We’ve been a hot team, and we’ve got to come and bring it every game.”

Smart was initially expected to miss just two weeks, but that timeline was soon altered to shoot for the first game after the All-Star break, which would put him back on the court next Friday in Detroit, a month to the day from his last appearance.

“We’re going to examine it when I get back,” Smart said of his hand injury. “Things are looking good. We’re going to meet with the team doctors when I get back and go from there and see how it goes.”

So how much does he want next Friday?

“Huge. Huge,” he said. “I mean, as a competitor, you want every game, especially when you’re coming off an injury, you want to be able to come back and be useful and help the team win. So I’m excited and I’m ready to go.”