Boston Coach Joe Mazzulla, right, talks to official Scott Twardoski during the second half of Monday’s 111-109 loss to the Houston Rockets. David J. Phillip/Associated Press

The Boston Celtics knew this six-game trip – with several beatable opponents – was important as they try to build good habits and fix some of their glaring issues as the playoffs loom.

But if Saturday’s win over the Hawks was an encouraging sign, Monday was a major step in the opposite direction.

The Celtics have suffered some puzzling losses to inferior opponents this season and Monday’s defeat is easily among the worst of the season, especially at this stage of the year. The Celtics didn’t start the night with the right energy and their bad habits and deficiencies were on display in ugly fashion in a costly, inexcusable 111-109 loss to the Houston Rockets, who are last in the Western Conference with 52 losses and already officially eliminated from the playoff picture. The Celtics pummeled the Rockets by 24 in December.

Forget the No. 1 seed. At this rate, the Celtics will be fighting over their final 13 games just to maintain the No. 2 seed, with just a one-game lead over the 76ers.

Jaylen Brown scored 43 points on Monday, and it would have been worse without him. The Celtics trailed by 11 with less than five minutes to go and fought back, but couldn’t complete the comeback. Down two with seven seconds left, Coach Joe Mazzulla drew up a play and put the ball in Jayson Tatum’s hands, but he missed a layup at the buzzer.

The Celtics were atrocious from 3-point range, finishing 12 for 42 (28.9 percent). They fell to 11-17 this season when shooting less than 35 percent from long range. But, Mazzulla was focused on the margins that continue to cost the Celtics games.

The Celtics were once again outhustled on the glass, where they gave up 15 offensive rebounds and 17 second-chance points to the young and athletic Rockets. They were beaten at the free-throw line. And they coughed up 13 turnovers, including four each from Tatum and Marcus Smart.

“Those are concerning, the margins,” Mazzulla told reporters in Houston. “That’s playoff basketball at its finest, is the ability to win those situations, so it’s concerning that we’re inconsistent in that. We have to be committed to those regardless of who we’re playing, regardless of the situation, regardless of how many games are left. It doesn’t matter. You have to be committed to those.”

AS RECENTLY AS last month, the Celtics looked like a team that was a lock for a top-two seed in the Eastern Conference. An ugly stretch of basketball compounded by Monday’s loss to the Rockets opened the door for a drop in the East playoff picture for Boston.

The Houston loss snapped a two-game winning streak and dropped the team to 2-4 in their last six games. The setback, along with a win by the Bucks over the Kings in Sacramento, has created a sizable gap once again in the standings for the top seed. Milwaukee now leads Boston by 2 1/2 games with 13 games left to play. The two teams meet once again in Milwaukee in a game that will decide the head-to-head tiebreaker.

The Celtics have more things to worry about at the moment than catching the Bucks. Philadelphia has won five straight games, pulling them one game behind the Celtics in the standings. The teams are tied in the loss column (22) but the Celtics have two games in hand. Boston does own the head-to-head tiebreaker after winning the first three games of the season series.

The teams will face off one more time in April in the final week of the regular season. The 76ers have a tougher schedule to finish the regular season (fifth hardest in NBA) but they are playing far better basketball than the Celtics over the past few weeks, putting Boston in danger of falling to the No. 3 seed and losing out on potentially holding home-court advantage in the second round of the playoffs.

The New York Knicks or Brooklyn Nets look like likely first-round matchups for Boston if they fall to the No. 3 seed. If the Celtics land a top-2 seed, the play-in tournament will determine their first-round opponent.

Despite Boston’s uneven play, Smart is not panicking about the team’s recent bumps in the road.

“I would rather (the struggles) happen now than later,” Smart told reporters in Houston. “I’d rather it happen now than when we get into the playoffs and it happens.”