Brown was named an Eastern Conference All-Star reserve earlier in the day - earning his first selection - but Tuesday's loss dampened the occasion.
"I don't feel very much like an All-Star because we're below .500 ... I think this is the most I've lost since I joined the Celtics," Brown said postgame, according to ESPN's Cassidy Hubbarth.
The Celtics are now 15-16 after Luka Doncic's two late threes handed Brad Stevens' team a seventh defeat in 10 outings. Boston sits in sixth ahead of Wednesday's test against the Atlanta Hawks.
Despite the team's poor run of form, both Brown and teammate Jayson Tatum were selected among the seven Eastern Conference reserves for the March 7 midseason exhibition in Atlanta.
Tatum, who was tabbed for a first All-Star appearance last season, shares the sentiments of Brown.
"It's like, damn, look at our record. It's hard to really focus on (the All-Star Game) when we're not where we want to be as a team," Tatum said, according to the Boston Globe's Gary Washburn.
"It's two-fold ... I was happy for (Jaylen) and then I was like, damn, you look at our record, it's hard to really focus on that when we are not where we want to be as a team," Tatum added.