Darlington have secured a top-seven finish in League Two and will begin their own play-off campaign next month.
Andrew Hughes, Darren Kenton and Frazer Richardson have all played at left-back for Leeds this season, and Ian Westlake's return from his loan spell at Brighton will add another left-sided player to the squad.
Westlake is expected to link up with United after making his final appearance for Brighton against Swansea City this weekend.
McAllister said: "It's obviously something we've had a close look at, but my feeling is that I'll probably leave Ben at Darlington for the play-offs. He's had a good little run up there and we've got Andrew Hughes and Darren Kenton who can play in that position. Westie's coming back from Brighton as well so there are a few guys who can cover for Alan."