When asked about the forward's status for Game 6 on Thursday, Carlisle responded, "I would like to be more optimistic than I am," via Tim MacMahon of ESPN.
Porzingis has missed each of the last two games with a sore knee, and the Mavericks are on the brink of elimination, down 3-2 in a first-round series with the Los Angeles Clippers.
After the 25-year-old was ejected in Game 1, he made a significant impact in each of the next two games with a combined 57 points and 20 rebounds.
He also averaged 30.5 points, 9.5 rebounds and 1.5 blocks per game during the seeding games in Florida.
Luka Doncic carried the Mavericks to an overtime win in Game 4 with Porzingis out, but Dallas was blown out 154-111 in Game 5 on Tuesday.
If Porzingis misses another game with his injury, it could put a lot of pressure on Doncic and the Mavs to keep their season alive. Dallas is already an 11-1 underdog to win the series, according to Caesars Sportsbook.
Maxi Kleber and Boban Marjanovic will have to play bigger roles in the front court with Porzingis unavailable.