Gary Johnson's men sealed their passage into round two before losing their prolific frontman to a right leg injury following a 69th-minute challenge with Millers man Danny Harrison at London Road on Tuesday night.
Rotherham, decimated by injuries and the unavailability of a couple of loan players, needed only nine minutes to spring a surprise by taking the lead through Marcus Marshall, who seized on an errant clearance from Nathaniel Mendez-Laing and drilled a shot through the legs of goalkeeper Joe Lewis.
But Posh conjured a turnaround late in the opening half thanks to two goals in the space of five minutes as George Boyd went from provider to goalscorer.
He delivered the outswinging corner from which record signing Ryan Bennett headed Posh level in the 34th minute and then fired the hosts ahead with a stinging low shot from just outside the box after collecting a short corner from James Wesolowski.
Mclean slammed into the roof of the net after being picked out by Mark Little and Posh added a fourth goal five minutes from time when an out-stretched Mackail-Smith prodded home a low right-wing cross from Mclean's replacement Lee Tomlin.