Terzic has landed on his feet in Bilbao and quickly become a favourite among supporters after two pre-season outings, the latest of which saw his side put eleven goals past Leioa without reply.
"We're happy because I think we did a lot of things well and managed to score goals," Terzic said in his post-match press conference, reflecting on the rout.
He also praised the progress shown by his squad since taking charge.
"Seeing the progression from Wednesday to Friday is important. In training we work in tight spaces, in matches there's more room, and we're pleased with the progression," he said.
Asked about the clear gulf in quality between his side and their lower-league opponents, the former Borussia Dortmund coach played down any concerns.
"The level gap isn't a problem. We wanted to start training very early so the players had their own time to develop ideas. We won't score that many goals every day," he said.
Terzic also touched on the physical condition of his squad, with the club carefully managing training loads as players build up their fitness.
"Both in training and today, they're working hard on that aspect. The fans? I'm feeling good energy, and that's very important for a new coach, I'm very grateful to the fans," he said.
Terzic was appointed Athletic Club head coach in May on a contract running until 2028, arriving after a spell at Borussia Dortmund that included a Champions League final appearance. His side's next pre-season test comes against Sestao on 22 July, as Athletic continue their build-up to the new La Liga campaign.
The win over Leioa, a Tercera RFEF side, followed an earlier pre-season victory and continued a pattern of comfortable results for Athletic as they ease into their preparations. Terzic used the friendly to rotate heavily, fielding a different starting eleven in each half as he looks to assess his options across the squad before the campaign begins in earnest.
Several first-team players were unavailable for the trip, either through injury or because they were still on duty at the World Cup, meaning a number of fringe and reserve players were given the chance to impress their new coach.
That depth of opportunity has been a feature of Athletic's pre-season so far, with Terzic keen to give every member of his squad minutes to work with before the more serious business of the league campaign gets underway next month.
For a club with such a strong sense of identity and history, Terzic's early impact on and off the pitch appears to have gone down well with supporters, who turned out in numbers to watch the rout unfold.
The German has been quick to acknowledge that support, repeatedly pointing to the atmosphere around the club as a source of motivation as he settles into his new role.