After surging to their fourth victory in a row last week, the Celtics were greeted with an appetizing upcoming schedule that included one game against the Lakers and two against the Nets. A more meaningful winning streak seemed quite possible, if not probable.

But these Celtics have made a habit of looking excellent against strong teams and meek against bad ones. Several players have acknowledged that when they face an opponent with a dismal record they do not ignore it, even though they understand that they should.

The Celtics lost to the Lakers in Kobe Bryant’s farewell to Boston on Wednesday, and then on Saturday returned to TD Garden and inexplicably fell to the Nets, 100-97. They will probably not have a more disheartening two-game stretch this season.

The Nets and Lakers have a combined record of 17-50, but they are 3-1 against Boston.

“We’re definitely not one of the best teams in the NBA, so I don’t get how we could possibly think it’s OK to play down to anybody,’’ Celtics point guard Isaiah Thomas said. “When we don’t play hard and we’re not the aggressive team, we’re one of the worst teams in the NBA. But when we bring that aggressiveness and we’re having fun and we’re defending, we’re one of the top teams in the NBA. I think a lot of people see that. So, somehow, some way, we’ve got to become more consistent.’’

Afterward, Celtics coach Brad Stevens said his team had been too passive and that he planned to “reassess things.’’ He said he needed to hold his players more accountable, and when pressed on his concerns about accountability, he pointed the finger at himself.

“This is more on me,’’ Stevens said. “We’ve had multiple times where we’re just not playing to a standard we need to play to. We’ve had multiple practices where certain guys have really looked good and stood out because others haven’t, and that’s just bad coaching. So I’ll get myself straightened out and I’ll work on what needs to be changed.’’

The Celtics trailed, 98-90, with one minute left and their hopes were dwindling by the second. But after Brook Lopez missed a shot, Thomas raced the other way for a layup.

He then came up with a steal and ultimately fed Jae Crowder for a layup as he was fouled. The free throw with 39 seconds left sliced the deficit to 98-95, and somehow, the Celtics had a chance.

Thomas and Crowder trapped Shane Larkin after the ensuing inbounds pass, forcing him to call a timeout. Stevens instructed his players to trap in the backcourt but to drop back into defensive position once the Nets crossed midcourt.

Instead, the Celtics trapped and went for a steal after Brooklyn had advanced the ball, and the Nets made three quick passes that led to an open dunk for Lopez.

“It was a mistake on our part,’’ Stevens said. “They were on their heels and so rather than just being back in the halfcourt, my kind of gut tells me, ‘Keep them on their heels and then pull back.’ And we just didn’t execute it well.’’

Still, the Celtics had a chance. Evan Turner scored on a putback with 11.8 seconds left to make it 100-97. After a timeout, Marcus Smart pilfered the inbounds pass and the Celtics rushed the other way. Turner, who has struggled with his long-range shooting this season, passed up a 3-pointer and found Jonas Jerebko in the left corner, but his open trey caromed off the rim.

“When we’re not the more aggressive team, we play like we did today,’’ Thomas said. “I think that’s plain and simple. Offensively and defensively we didn’t hit first, and I don’t know why we didn’t keep doing that.’’

Lopez was dominant, scoring a season-high 30 points and adding 13 rebounds. The Celtics went to a small lineup for almost the entire fourth quarter, and Lopez made 4 of 7 field goal attempts in his nine minutes.

“It was just the way Boston went,’’ Lopez said, “so we tried to take advantage of that.’’

Thomas led the Celtics with 24 points and six assists and Crowder added 16 points and seven rebounds. Celtics guard Avery Bradley suffered a hip contusion in the second quarter and did not return, and his status for Monday’s game against Brooklyn was not immediately known.

The Celtics shot just 38.6 percent from the field overall and made 7 of 32 3-point attempts.

With 9:56 left in the third quarter and the score tied at 58, Lopez attempted a shot inside and Celtics forward Jared Sullinger pulled him to the ground by his shoulders.

Nets guard Jarrett Jack took exception to the foul by Sullinger, who also fell to the ground, and shoved him. Crowder then shoved Jack, and the two had to be separated. Sullinger was whistled for a flagrant-1 foul and Jack and Crowder both received technical fouls.

Following that play, the Nets seized control with a 9-0 run and stretched their lead to 11. But with 3:06 left, Jack took a bad step on a fast break and crumpled to the floor holding his left knee. His teammates rushed out to him and he was helped off the court.

The Nets led, 77-66, at the time of Jack’s injury. The point guard had nine assists in 23 minutes, and when he exited, Brooklyn’s offense appeared to lose its flow. A fadeaway jump shot by Turner from the baseline with 8:38 left capped a 17-6 Boston run and tied the score at 83.

But the Celtics could not complete their final push and had no answer for Lopez, who methodically went to work in the post time after time.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.