The Heat made the announcement shortly before tipoff of the win-or-else game against the San Antonio Spurs, who took a 3-1 lead into Sunday night's matchup. It's Allen's first start in 43 postseason games with the Heat.
Chalmers had started every game in which he's appeared with Miami over the last three seasons. The last time he played as a reserve was in the 2011 NBA Finals against Dallas.
Chalmers entered Game 5 averaging 3.5 points in the series, shooting just 5 for 18 from the floor.