The Serbia international has been in irresistible form since moving to St Mary's and Koeman says he never had any doubts about his ability.
In fact, considering how highly he rated him, the Saints boss insists the only doubts he harboured were whether he would be able to sign Tadic in the first place.
“I was always confident about Dusan because he has exceptional qualities. His assists, his goals, his feel for the game belongs on the big stages," Koeman is quoted as saying by the Daily Mirror.
"But I didn’t expect to sign him for Southampton, I was surprised when he came here. I thought we would be fighting against bigger clubs.
“But Dusan, by himself, decided it was better to take the first step up and maybe he can think about taking another one later. That’s also a good quality – he has good sense.”
Koeman's words came before Southampton hosted Stoke City at 3pm this afternoon, when they were hoping to maintain the sensational Premier League form that had taken them to third in the table.
In four days' time, the two sides will meet again when Stoke host the Saints at the Britannia Stadium in the Capital One Cup.