"By the time we got to the playoffs, I don't think he was ready," Riley said Monday, six days after the Heat were eliminated by the Sixers in five games. "He wasn't in great shape. He wasn't fully conditioned for a playoff battle mentally. He and we got our heads handed to us."
Whiteside also needs to make major changes and repair his relationship with coach Erik Spoelstra, according to Riley.
Whiteside played a total of just 76 minutes in the five games against Philadelphia and struggled to contain Embiid, who averaged 18.7 points, 10.3 rebounds and 3.0 blocks per game despite missing the first two games of the series.
The 7-foot Whiteside, who has two years remaining on his four-year, $98.4 million deal with the Heat, publicly complained about his role and his lack of playing time after Miami's loss in Game 3.
"I'm not going to give him any kind of excuse," Riley said. "But (Whiteside's) season started with an injury, and all year long there was a dilemma of some kind."
Whiteside dealt with injuries during the season and didn't want to wear a knee brace that the Heat insisted upon. Riley also said that if Whiteside and Spoelstra need an intervention to solve their relationship issues, he'll handle it.
"The disconnect between (Whiteside) and Spo, that's going to take a discussion between them and it's going to take thought on the part of coach and also Hassan," Riley said. "How will Hassan transform his thinking — 99 percent of it to get the kind of improvement that Spo wants so he can be effective? How can Spo transform his thinking when it comes to offense and defense and minutes or whatever?''
Whiteside, 28, had his worst game of the series in Philadelphia's series-clinching victory in Game 5 last Tuesday, when he scored just two points in 10 minutes.
Spoelstra came to Whiteside's defense three days later during his end-of-season news conference, saying that Whiteside "battled and grinded" through his injuries during the season. Spoelstra also said that the "narrative and storylines" about Whiteside are "unfair."
Information from The Associated Press was used in this report.