Whiteside injured the knee early in the second quarter Saturday and missed the rest of Game 3. He is day-to-day, according to the Heat, but the typical recovery time from such an injury is at least a couple of weeks.
The severity of the sprain was not announced.
Whiteside attended Sunday's practice but was not seen during the portion of the workout that was open to the media. He was not made available to reporters to discuss his injury.
"He had an MRI today that went about as well as could be expected," Heat coach Erik Spoelstra said Sunday. "Where my mind was, where Hassan's mind was leaving this building [Saturday night], that's about the best news that we can have. He's just going to be doing treatment and a lot of rest."
After landing awkwardly on an offensive rebound, Whiteside limped off the floor and immediately underwent X-rays, which came back negative.
The Raptors also lost their starting center for the remainder of Game 3 as Jonas Valanciunas suffered a sprained right ankle in the third quarter.
Whiteside had been hampered by a right knee strain after another awkward landing in Game 1 on Tuesday and had to be removed from the game. He returned shortly afterward and logged 39 minutes in the overtime victory. In the Heat's Game 2 overtime loss, Whiteside played 39 minutes again, marking the first time in his career that he had played at least 36 minutes in consecutive games.
He scored six points before leaving Saturday's game with the Heat trailing 25-19. The 26-year-old has posted a team-high 24.0 player efficiency rating this postseason, leading all players with 110 rebounds.
"It's unfortunate," Heat guard Dwyane Wade said Sunday of the injuries impacting the series. "When it comes to injuries, it's not always about who's the best team; it's always about who's the healthiest team and what impact injuries make on a team. There's nothing you can do.
"It'll be a different game without Hassan, but we have to figure out a game that's successful for us to get a win. There are no excuses from our side. We lost an important piece to our team. But we have to find another important piece to step up."
Toronto leads the series 2-1.
The Heat center will be an unrestricted free agent this summer after spending the past two seasons with the Heat and finishing third in this season's NBA Defensive Player of the Year vote behind San Antonio Spurs forward Kawhi Leonard and Golden State Warriors big man Draymond Green.
The Whiteside injury comes on the heels of the Heat announcing Wednesday that big man Chris Bosh will miss the remainder of the postseason after being sidelined with an unspecific medical condition since February.
The Associated Press contributed to this report.