Flores didn't get into specifics but said the issue came as a surprise.
"He has my support. Things happen," Flores said. "When they happen, they're not part of normal every day, it's a surprise from that standpoint. He's dealing with it. We'll support him."
Fuller, 27, was set to make his debut with the Dolphins after serving a one-game suspension to start the season. He signed with Miami in March after spending the first five seasons of his career with the Houston Texans.
He practiced Wednesday but was listed as absent from Thursday's practice while attending to his personal matter, which Flores said would hold him out of Friday's practice as well.