The striker, who scored 47 goals in 93 appearances for the Dons, completed a reported £2 million move to Birmingham City on Sunday afternoon.
The Dons are hoping to sign Scotland Under-21 top goalscorer Fraser Hornby as an immediate replacement on loan from Stade de Reims.
"He is certainly a player we really admire and we think is a good young player and would be a good addition," Docherty said.
"You are never there until you are there. We are endeavouring to do all we can as club to get things over the line."
On Rangers' immediate pursuit of their pre-contract signing, Scott Wright, Docherty said: "I can't comment on that, other people are working on that as we speak, purely speculation at the moment."