The team said a second MRI confirmed the sprain, which Lillard sustained in the third quarter of the Blazers' 135-115 loss to the Lakers in Monday's Game 4. The five-time All-Star selection underwent an initial MRI after the game, but the team said the results were inconclusive.
Lillard, 30, scored only 11 points on 3-of-9 shooting in 27 minutes Monday. He appeared to injure his knee after planting his right foot on a drive to the basket.
Lillard, who also was playing through a dislocated left index finger, was the unanimous winner of the MVP award during the NBA's seeding games inside the bubble. He has averaged 30.5 points per game since the restart -- second only to Houston Rockets star James Harden.
The Lakers hold a 3-1 lead and can close out the Western Conference series in Wednesday's Game 5.
Meanwhile, Porzingis will miss Tuesday night's Game 5 against the Los Angeles Clippers because of right knee soreness. The Mavericks initially listed him as questionable before downgrading him to out.
It will be the second straight game Porzingis misses due to the knee ailment. He was a late scratch before the Mavs' Game 4 overtime win Sunday.
Porzingis, who was ejected during the third quarter of the Mavericks' Game 1 loss, has averaged 23.7 points and 8.7 rebounds in the first playoff series of his career.
The best-of-seven series between the Mavs and Clippers is tied at two games apiece.