The midfielder has had a recurrence of his medial knee ligament injury this week and full-back Martin Olsson will be sidelined for a month after a ball struck him in training and caused internal bleeding in his eye.
Colin Kazim-Richards added his name to the casualty list, limping off with an ankle problem early in the second half to deal a further blow to Rovers' play-off hopes.
Blackburn sit seven points behind sixth-placed Brighton in 10th, a point and a position shy of Leeds, but Appleton believes they could mount a late charge following a fifth clean sheet in his ninth game at the helm.
"When I walked into the football club nobody had any thoughts whatsoever about getting into the play-offs," he said.
"I think a lot of the damage was done before I arrived but, saying that, there's 13 games to go.
"The message to the players after the game was you just need to keep knocking on the door.
"If you keep knocking on the door and asking questions hopefully someone will slip up, and generally someone does.
"There's still a lot of games to go and if we're still in and around it with six or seven games to go, that's the time to really start putting the pressure on."