The Belgium international has endured mixed form since joining the Reds from Sunderland in 2013, but the 29-year-old did manage to provide the club with a strong run of performances during the back-end of the 2016-17 campaign.
Klopp's keeping options have been bolstered by the recent return of Danny Ward from Huddersfield Town, who will provide further competition for second-choice stopper Loris Karius, although Klopp has insisted that Mignolet is to keep his place as the club's number one for now.
"It is Simon. It was a battle at the highest level in all the games in pre-season," Klopp told the club's official website.
"Simon looked really stable. He was involved in all the sessions. He did not have one bad day, which is cool, and after the season he had last year he is very confident. He has improved and is still improving in parts of his game.
"Loris had a few little issues but was really strong in the games he played. Wardy had a bad pre-season because of back problems.
"He couldn't train and for a goalkeeper it is even worse to have back problems than another player, but everyone could see against Atletico that we have another good goalkeeper. In this moment it is Simon one, Loris two and Wardy three. That is not a decision for the next 20 years."
Liverpool begin their new Premier League campaign early on Saturday afternoon when they travel to face Watford.