The Suns still had to finish well and they did by scoring the last seven points for an 87-80 win Friday night that moved them one game behind Memphis for the eighth and final Western Conference playoff spot.
"Our offense wasn't very good," Phoenix coach Jeff Hornacek said. "Defense, when you aren't shooting the ball well, can win you the game and I think it did tonight."
The Suns scored just 58 points in the last three quarters. But the Celtics made a season-low 30.9 percent of their shots and missed 65.
Boston coach Brad Stevens was especially upset with his team's play after Jeff Green's two free throws for the Celtics made it 80-80 with 2:02 left, the fourth tie of the fourth quarter.
"We didn't play with any poise," he said. "The last 40 seconds were awful."
Alex Len started the decisive spurt with a three-point play on a tip-in and a free throw with 54.1 seconds left. The Suns (37-28) finished the scoring with a dunk by Markieff Morris and a layup by Goran Dragic, who led them with 20 points.
Before the season people said "we were going to have 15, 16 wins," Dragic said. "We don't care if it's pretty or ugly. We need to have those wins."
Eric Bledsoe had 17 points and 10 rebounds and Morris added 15 points for Phoenix.
Kris Humphries led the Celtics with 11 points, 13 rebounds and one thrown towel while on the bench when they collapsed in the final minute.
"He's upset because we lost and he certainly has got a right to be," Stevens said. "That was not a good ending. It was not a good performance."
Chris Johnson also scored 11 for the Celtics, but threw a bad pass that was stolen by P.J. Tucker and led to the dunk by Morris with 23.1 seconds to play.
The 7-foot-1 Len put the Suns ahead to stay after entering the game in the fourth quarter when Miles Plumlee went out after being poked in the eye. The rookie center was averaging just 2.4 points and 9.1 minutes per game before Friday but finished with six points and three rebounds in nine minutes.
"Obviously, the tip was huge," Hornacek said, "but I think (with) just his size in there, he does a nice job of sliding his feet and getting in a good position, making guys shoot over him."
The Celtics lost for the 10th time in 13 games while shooting worse than their previous low for the season of 32.3 percent in a 109-85 loss to the Houston Rockets on Nov. 19.
They trailed 64-60 after the third quarter, but scored the first nine points of the fourth to take a 69-64 lead on Avery Bradley's layup.
Then the Suns went back in front, 70-69 with six straight points, but the Celtics pulled ahead 78-75 with 3:17 remaining before the Suns tied it on Bledsoe's three-point play.
The Suns led by 10 in the first quarter and by 11 when they took a 38-27 lead with 8:34 left in the second quarter.
Then a layup by Rajon Rondo began a 17-6 run by the Celtics that tied the game at 44 with 59 seconds remaining in the half. Two free throws by Channing Frye with 26 seconds to go put Phoenix on top 46-44 at intermission.
Both teams went cold from the field in the third quarter. Boston made 4 of 20 shots and Phoenix hit 7 of 21. But the Celtics trailed by only four after Kelly Olynyk got an offensive rebound and sank a short jumper from the left baseline at the buzzer.
Their fourth-quarter finish was much worse.
"We kind of got a bit antsy there at the end," Olynyk said. "I think we kind of rushed a bit."
Bradley missed his previous 13 games with a right ankle injury. ... Bledsoe played his second game after missing 33 following knee surgery. ... The Suns scored just 17 points in the second quarter and 18 in the third. ... Former Celtic Gerald Green scored four points and has 12 in his last two games after averaging 26 in his previous five.