Riley told reporters the team is no longer working toward Bosh's return.
"We are not," Riley said in his office at AmericanAirlines Arena. "I think Chris is still open-minded. But we are not working toward his return. We feel that, based on the last exam, that his Heat career is probably over."
The Heat announced Friday that the 11-time All-Star would not be allowed to participate in their preseason training camp after failing a physical and remaining unable to return to basketball activities.
The Heat declined to explain why Bosh failed the physical and cited a clause in the collective bargaining agreement that prohibits the team from releasing certain medical information without a player's consent.
Asked whether he felt Bosh's NBA career is over, Riley said, "That's up to him."
The 32-year-old Bosh wants to resume playing even though each of his last two seasons was cut short by blood clots. He was examined by doctors last week in an attempt to be cleared for camp with the Heat, who were flying to the Bahamas on Monday for Tuesday's start of training camp.
Riley said Bosh will not be on that trip.
"It's pretty definitive from us, in our standpoint, that this is probably going to be a time where we really have to step back," Riley said before the start of the team's media day. "His health, playing and economics -- it's been health, health, health. Whatever the cap ramifications are, they are there, but we never ever thought about that."
The Heat would receive salary-cap relief going forward on Feb. 9 if Bosh is ruled medically unable to play by an NBA specialist.
If the medical issue keeps Bosh out of action for good, he would be paid the remainder of his contract -- three years, $76 million. He averaged 19.1 points and 7.4 rebounds in 53 games last season when the second bout with clots began in February.
Bosh released a video statement Friday night that was accompanied by a Twitter post of, "Setbacks may happen, but my intentions remain the same. Thank you all for the warm wishes and support."
"Just because the journey has ups and downs doesn't mean I will stop sharing with you guys," Bosh said in the video. "So I will just continue to share despite what's going on."
Coach Erik Spoelstra said the situation with Bosh has been difficult.
"I love C.B. dearly," Spoelstra said Monday. "It was tough to watch C.B. and his family go through this the last couple of years. Your heart just goes out to him.
"I'm going to prepare for this group right now. But there's going to continue to be changes -- hopefully not as big of an impact as this. The other 19 players that are going to training camp, my focus will be on them. That does not take away my feelings on C.B."