McKennie sustained the injury in Juve's Champions League round of 16 clash with Villarreal last month. Sources previously told ESPN he suffered fractures to the second and third metatarsals in his left foot and was expected to be out for between eight to 12 weeks.
However, following Juve's 1-0 win over Spezia in Serie A, Allegri said McKennie will not play for the remainder of the season.
"The season is over for Weston because of his injury," Allegri said. "He'll be back next season."
The injury comes at a critical time for McKennie. The midfielder had been in good form for several months, emerging as a key figure in Allegri's plans.
The U.S. is in second place in CONCACAF's final-round Octagonal for World Cup qualifying. Should Gregg Berhalter's side finish in fourth place, outside the top-three automatic qualification places, then they would play in the intercontinental playoff in June.
McKennie, who scored three goals in 20 league games for Juventus this season, is expected to be available when the 2022-23 Serie A season begins in August. The World Cup at Qatar is set to begin Nov. 21 and will run into December.