A team spokesman later noted the X-rays came back negative. Rivers previously stressed that Bradley's condition hasn't worsened at all, but that the lack of improvement prompted another test.
"They just want to make sure (his ribs aren't) cracked or something like that. I think the X-ray showed it wasn't, but maybe do a deeper one just to make sure," he said.
Rivers said Bradley could still travel with the team for the upcoming two-game road trip, but listed him as doubtful for Sunday's matchup with the Pistons.
"It just didn't improve," Rivers said. "He was better. He told me he felt much better and then today he said, 'Man, I just feel the same,' so that's not an improvement."
Bradley made his season debut on Jan. 2 after recovering from offseason shoulder surgeries and helped lift Boston to a 6-2 record over the eight games he played in before missing Friday's matchup with Chicago.