Gabriel Zakuani.
Posh again looked uneasy at the back in front of their home fans and cover was non-existent as Rovers' Ian Goodison swung over a seventh-minute centre for unchallenged Chris Greenacre to send a back header wide of rooted home goalkeeper Joe Lewis.
Greenacre then missed a great opening, lifting the ball over the bar from four yards after Lewis had done well to block a drive from Edrissa Sonko, but made no mistake when the Posh's back line was caught square on the half-hour, collecting a Bas Savage header and rounding Lewis.
It could have been so different for Posh with top scorer Craig Mackail-Smith hitting the far post from an acute angle with the game barely three minutes old, Ben Chorley hooking clear as the ball rebounded along the line.
Mackail-Smith gave Darren Ferguson's side a much-needed lifeline a minute before the break, firing his sixth goal of the season into the roof of the net after meeting George Boyd's driven centre.
Danny Coyne twice went down to deny Mackail-Smith as Posh chased an equaliser in the second half and the ball squirmed wide when Russell Martin's shot took a deflection off defender Chorley.
With Steve Jennings picking up anything loose in midfield. Rovers still threatened and, after Ryan Shotton had glanced Andy Taylor's corner wide, Savage fired a shot just the wrong side of Lewis's left-hand post.
Posh stuck to their task and Zakuani, making a home debut on loan from Fulham, levelled the scores with a 77th-minute header from Martin's corner.