Haller, who returned in January after six months' treatment for testicular cancer, scored his first goal for Dortmund in his side's 5-1 home win over Freiburg on Saturday.
Speaking ahead of Wednesday's German Cup clash at local rivals Bochum, Terzic said the club's doctors and coaches were constantly in "open and honest communication" about how best to re-integrate the returning striker.
"It's a situation that none of us can really assess. Neither Sebastian, myself nor our medical department had any experience of something like this," Terzic said at the club's training base in Dortmund.
"When someone comes back from an ACL tear or a shoulder injury, we can make an estimation because we have experience there. But with this we are all inexperienced, so the solution is open and honest communication."
Terzic disagreed when asked if Haller could play the full 90 minutes against Bochum despite an apparent lack of preparation.
"I see it differently, he's had the longest preparation anyone could imagine.
"How hard he worked between his therapies... it's hard to comprehend how intensely (he worked)."
On Saturday, the France-born, Ivory Coast striker headed in a cross from Raphael Guerreiro midway through the second half for his side's third goal.
After the match, Haller said he was "waiting for this since day one."
"I'm floating on a cloud."
Dortmund CEO Hans-Joachim Watzke on Monday praised the striker and his work ethic, saying his return was "a miracle".
"The merciless consistency with which he has confronted the illness from day one is unbelievable," Watzke told Germany's Ruhr Nachrichten newspaper.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)