Team president Pat Riley expressed his belief Monday that the center wasn't ready for the postseason mentally or physically, per Tim Reynolds of the Associated Press.
Whiteside averaged 5.2 points and 6.0 rebounds in the first-round loss.
"He's going to have to do something to change, because he's a hell of a player," Riley said, per Reynolds.
After signing a four-year, $98 million contract ahead of the 2016-17 season, Whiteside responded with 17.0 points and a league-leading 14.1 rebounds per game, both of which were career highs. He took a step back this season, although his 14.0 points and 11.4 rebounds per game were still far better than what he managed in the postseason.
Meanwhile, the 28-year-old blamed the coaching staff for not giving him enough playing time.
"At least give me a chance to fight," he said, per Ira Winderman of the Sun Sentinel. "I can understand if I was playing 30 minutes and I played bad. At least give me a chance."
The center only saw 15.4 minutes per game against the 76ers.
Whiteside's problems with the coaching staff have been brewing for months.
Riley mentioned "Bracegate" as an issue that contributed to Whiteside's poor postseason, per Reynolds. After the big man missed 13 games due to a bone bruise, the Heat forced him to wear a knee brace, although he wasn't a fan of it. He bristled about his playing time and questioned head coach Erik Spoelstra's rotations during the regular season, too.
Whiteside has one more season under contract before he can choose whether to decline his 2019-20 player option, so he and the Heat must figure out a way to get on the same page moving forward.