Kelhar has been out for the duration of the season with a cruciate knee ligament injury, but he has returned to development squad action and Bullen has been impressed with what he has seen.
He says that the centre-back's attitude has been exemplary and Bullen believes that injuries could see him back in the first-team squad soon.
"Dejan is one of the most genuine and honest professionals you will meet," Bullen said. "With his attitude towards the game and the type of personality he has, he is a fantastic lad.
“It’s magical to see him back on the training ground, back in the games and getting involved. With Glenn [Loovens] and Kamil [Zayatte] out, Kelhar is slowly getting back and pushing to step out in the blue and white shirt before the end of the season.”
Kelhar's injury has delayed his Wednesday career from starting, as he has not been able to make an appearance for the club since arriving this summer.
Now he is returning to fitness he will be hoping to prove that he can have a role to play for Stuart Gray's men and show exactly why he was brought to Hillsborough.
Breaking into Wednesday's team could be a tough ask for Kelhar, though, as the side have been so strong defensively throughout the season.
Wednesday have conceded just 40 goals in their 39 Championship matches to date and Kelhar will need to make an immediate impression if he is to dislodge any member of their current back-four.