Harkless had the MRI after spraining it during the Blazers' 97-90 Game 2 victory on Wednesday.
Harkless, who started 53 of the 60 games he played during the regular season and all seven of Portland's playoff games, suffered the injury when he landed on the foot of teammate Zach Collins midway through the second quarter.
He was unable to return as the Blazers held on to even the best-of-seven series at 1.
After the game, Harkless told reporters he was day-to-day and uncertain for Game 3 in Portland. Jake Layman, who started when Harkless dealt with knee soreness during the regular season, got the call to begin the second half on Wednesday.
However, during a conference call on Thursday before the MRI results were known, Blazers coach Terry Stotts indicated that reserves Rodney Hood (who had 15 points in Game 2), Seth Curry and Evan Turner also are options if Harkless is unable to play.
"We'll talk about that [Friday] as a coaching staff," Stotts said.