Awoniyi brought an end of his six years relationship with the Reds after agreeing permanent switch to the German Bundesliga Club side this month.
Union Berlin paid €8million to sign the striker from Liverpool after a successful loan spell last season.
According to Awoniyi, Klopp backed his decision to return to the club, because they showed genuine interest in him.
“He said it was the most important thing for a young player,” Awoniyi told Kicker.
He spent time on loan with clubs in Belgium, Netherlands and Germany.