The big man is now considering surgical options and no timetable for recovery was provided, but Achilles tears generally sideline players for the better part of a year.
Powell suffered the non-contact injury while driving toward the basket in the first quarter of Tuesday's contest.
The injury is a serious blow to the Mavs, who currently sit fifth in the Western Conference. Powell has started 37 of 40 games at center this season, averaging 9.4 points and 5.7 rebounds. The 28-year-old could also miss out on playing for his native Canada at this summer's last-chance Olympic qualification tournament.
Mavs teammate J.J. Barea, who returned this season after rupturing an Achilles last year, said he expects Powell to bounce back.
"Nobody better or mentally stronger to come back from this than him," Barea told ESPN's Tim MacMahon. "Nobody has ever and will ever work as hard as he is going to do."