Adrian has been without a club since his contract expired at West Ham earlier this summer.
He spent six years with West Ham, racking up plenty of Premier League experience, but ultimately lost his place to Lukasz Fabianski.
Is Adrian a good replacement for Mignolet?
Adrian is a free agent and had been scheduled to have talks with Real Valladolid on Monday about a permanent move.
Mignolet left Liverpool on Sunday after he sat on their bench for the last time in the Community Shield match against Manchester City at Wembley, with Alisson Becker's status as the club's undisputed No.1 forcing the Belgian keeper to move on in search of regular first team football.
Mignolet, the Belgium worldwide, said his goodbyes to Liverpool's squad at Wembley on Sunday night before heading to Luton Airport to take a private jet to Brussels. The Reds have brought in two new players - Sepp van den Berg and Harvey Elliott - but both are expected to link up with the youth squads.
"This is important. He is a 'keeper who knows the Premier League, who has played in pressure situations and has the highest quality".