Montgomery was optioned to the team's alternate training site Tuesday.
The 27-year-old was expected to begin the season as the Yankees' No. 5 starter, but instead he won't be included on the 30-man roster to open the season. Montgomery delivered five scoreless frames during Sunday's exhibition contest with the Mets, so he finished summer training with a strong performance. New York has the benefit of an off day Friday after the season opener Thursday, so it appears manager Aaron Boone will take advantage with an extra reliever out of the gate. Montgomery could still join the rotation early in the season, but until he's recalled or the team provides more information, his immediate outlook remains uncertain.