A goal from Posh debutant Liam Dickinson at the start of the second-half proved to spur Jim Gannon's side to a fine win and Keane believes the setback proved crucial.
He said: "It's very, very disappointing. We lost our way and we didn't look like getting back into it during the second half.We needed the second goal, but we never got it.
"We conceded early in the second half, from our kick-off, and that gave the impetus back to the home team. After that we still didn't react quick enough.
"The first minute of the second half definitely changed the course of the game."