Coyle, manager at McDiarmid Park between April 2006 and November 2007, made nine changes to his starting line-up following the goalless draw at Falkirk on Saturday.
St Johnstone started brightest and created the better chances throughout but were unable to force home their dominance.
It was against the run of play when Petrov collected the ball down the right side of the box, cut inside and blasted a left-foot shot into the roof of the net after 76 minutes.
Four minutes later Elmander extended Bolton's advantage. The Sweden international received the ball in similar fashion to Petrov but beat the goalkeeper with a low shot.