The 23-year-old Olympian appears set to join Japan goalkeeper Eiji Kawashima at the Belgian outfit after passing his medical check and agreeing to a four-year contract
The club's second striker reinforcement following Iran's Reza Ghoochannejhad, Nagai is set to join the Belgian Pro League outfit through June 2017.
The 177-centimetre-tall attacker joined Grampus in 2011 after graduating from Fukuoka University. He played a key role in Japan's 2012 Olympic qualification, scoring two game-winning goals in London during Japan's fourth-place campaign.
A video interview accompanying the announcement showed Nagai undergoing medical checks and meeting with club doctors. Japan starting goalkeeper Eiji Kawashima, now playing his third season in Belgium, could be seen acting as a translator.
Nagai scored 10 goals in 30 appearances last season for Grampus, who finished in seventh place. The 2010 J-League champions are expected to receive as much as €1 million in return for the young star. The club has yet to make a public statement regarding the deal.