Wednesday enter tomorrow's home game with Birmingham City on the back of a three-game unbeaten run over the festive period.
The team's performances in that run have been a significant step up from previous months.
Despite that, caretaker boss Bullen insists there are still vast improvements to be made by the players.
"They are in a lot higher and confident state now," Bullen said.
"I don't think it was a lack of work rate or energy prior to Jos leaving.Â
"It is too easy to say that when results aren't going your way and to say 'they are not working, they are not running.'Â
"There was a drop in confidence and a drop in belief.Â
"That seems to be filtering its way back in.Â
"I don't think we are at the level we can be and the level we need to be at to really push on, but it is heading in the right direction.
"Our quality on the ball still has some way to go.
"I think, and the players will agree with me, that there's still room for improvement."
Bullen remains in the dugout for the time being, with Wednesday still attempting to thrash out a deal with Steve Bruce to become Luhukay's permanent successor.Â
Despite not knowing how long he will be in the hotseat for, Bullen admits he is relishing every day that he remains in situ.
"I don't see it as uncertainty - I just see it as a pleasure to have been given the opportunity," he added.
"Everybody knows my history with the football club and I'm just enjoying every minute of it.Â
"The games come thick and fast so you don't really get a chance to sit down and analyse everything.Â
"I'm just planning on Birmingham."