Kretzschmar shot into an empty net from 10 yards eight minutes from time to clinch all three points after fellow sub Jon-Paul Pittman's low drive had been parried by Rovers goalkeeper Steve Mildenhall straight into his path.
The home side were a man down from the 26th minute when David Clarkson was sent off for catching Wycombe midfielder Josh Scowen with a flailing arm as they battled for possession.
From then on the visitors had most of the ball without being able to force many saves from Mildenhall.
The Rovers 'keeper did well to block a close-range shot from Pittman just after the substitute's introduction in the 76th minute, but for much of the time Wycombe's shooting did not match their approach play.
Rovers were unable to create clear-cut chances of their own and had to battle hard to stay in the game for as long as they did.
John Ward's team, winless in seven league games, remain towards the wrong end of the table while back-to-back victories have moved Wycombe up to seventh.