According to the Associated Press (h/t USA Today), James suffered a right calf strain, and Irving is dealing with a knee injury "which the Cavs are calling tendinitis."
The AP report noted Irving's knee problem is on his surgically repaired left one.
Cleveland won't have Tristan Thompson available either because of a thumb injury, and Kevin Love was listed as questionable because of an illness.
The Cavaliers enter play Monday tied with the Boston Celtics atop the Eastern Conference at 51-29. Cleveland owns the tiebreaker, but a short-handed loss to Miami would give the Celtics the opportunity to seize the top seed and home-court advantage in a potential Eastern Conference Finals matchup.
Boston plays the 20-60 Brooklyn Nets on Monday.
On the flip side, this is a break for a Miami squad fighting for its postseason life. The Heat are tied with the Chicago Bulls at 39-41 for the last playoff spot in the East, although the Bulls hold the current tiebreaker.