The Celtics (18-34) couldn't keep pace with Dallas Sunday night, falling to the Mavericks, 102-91.
The Mavs (31-21) outscored the C's in each of the first three quarters and coasted to their 31st win of the season while the Celtics dropped their first game in over a week after winning three straight.
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Rajon Rondo played a season-high 37 minutes in the Boston loss and led the C's with 12 assists. Jeff Green scored a team-high 18 while Chris Johnson chipped in with 14 off the bench.
Dirk Nowitzki led Dallas with 20 points.
What they'll be saying: The Celtics once again looked like a lottery team after facing their first non-lottery team since the Kevin Garnett/Paul Pierce reunion at the Garden two weeks ago. The Mavs built on a steady lead in the third quarter and Boston couldn't hang with Nowitzki and the Mavs from there on out. The C's will be in Milwaukee Monday (8 p.m., CSNNE) to face the Bucks, the owners of the worst record in the NBA at 9-41. Losing to the Bucks would certainly qualify as a "quality loss" for those in Boston in full "tank" mode.