John Welsh put Rovers in front before Northern Ireland international McCann, restored to the starting line-up after two matches on the bench, levelled matters.
McCann had a hand in Posh`s winner as his corner brought an own goal from Enoch Showumni.
Ferguson said: "We have a lot of players in our dressing room capable of turning a game and Grant did just that. He is a terrific player who is always capable of scoring and creating goals, just as he showed.
"I settled on my shape and system for the game before we had so much rain and in hindsight we probably had too many creative players out there.
"We probably tried to play too much football in the first half but after the break we we won more balls, turned Tranmere round and got our rewards."
Source: PA
Source: PA