Fati, 17, scored twice in the first half as Barca closed the gap to league leaders and rivals Real Madrid to three points.
He became the youngest player to score a brace in LaLiga in the 21st century, breaking a record previously held by Juanmi Jimenez.
Fati has netted five goals this season and Setien hailed him for stepping up for Barcelona, who are set to be without the injured Luis Suarez until the end of 2019-20.
"I think that Ansu is a boy that is emerging, that we all know that has great potential," head coach Setien told a news conference.
"You have to understand that you have to give him peace of mind and confidence to exploit what he has inside.
"Apart from the two goals, what I give credit is the work he has done in defense. He has worked hard. He has given us solutions."
Fati struck in the 30th and 31st minutes and Barcelona closed out their win despite Ruben Rochina's late goal for the visitors.
Setien was satisfied with his side's performance, although he expects further improvement.
"I am happy with many things, but not with everything. We have given up too many shots," he said.
"We made a very good first half, I think we could score more goals. We lost a little control in the second half and conceded too much.
"But it was a good job by the team."
Barcelona are in action again on Thursday, visiting Athletic Bilbao in the Copa del Rey quarter-finals.