If Dortmund win today, would Terzic be the 3rd/4th worst winning manager of all time in the CL/European Cup?
Red Star Belgrade's manager Ljupko Petrovic in 1990/91 will remain worst IMO, he really had a career year that season (he was in charge only for that one season), but it was really because of the some great players the team had. He bounced around a lot after that (including another stints with Red Star), but at least won some other titles in lesser leagues.
Although I think you could reasonably argue Aston Villa (winners in 1981/82) manager Tony Barton to be worst. It was before my time, so personally can't really judge him. But outside of that final win, his career was rather terrible. He wasn't even in charge of Villa (Ron Saunders was) during the previous season, which earned Villa the qialification for European Cup for the next season when they won it. He only became manager 3 months before the final. He then had two average season in charge of Villa (finishing 6th and 10th place in the league) before being sacked. Soon after he had a heart attack and his managerial career was over.
So not that different from Di Matteo, but still somewhat worse career.