Wycombe took the lead in the 13th minute at Adams Park on Saturday through Matt McClure but the hosts were given a major boost when Diamond was dismissed after he was judged to have bundled over McClure and Joel Grant rolled in the resulting penalty. McClure made the League Two points safe six minutes later when he slammed in a rebound.
A disappointed Rowett said: "It was pretty awful with 10 men but whatever way you look at it, the referee has changed the game.
"I didn't agree with the decision for the penalty and definitely not for the sending off but I felt after that that we should have got to grips with the game.
"First half we weren't quite good enough but we still got into some good areas and put some balls into the box that we needed to do more with and forced eight or nine corners.
"We needed to capitalise on that but one decision can change an awful lot of things. Our performance wasn't good enough - again - and that's two games in a row where we needed to do so much better."