Moses Odubajo is trying to earn a new contract from Brentford.
The defender met up with Celtic during their pre-season training camp in Austria but after talks with Brendan Rodgers he returned to England.
Odubajo last played a competitive game in May 2016 when he helped Hull to a Premiership Play-Off victory over Sheffield United.
Although he was offered a new contract by Hull he rejected that option with Celtic keen to have a look at the 25-year-old.
Rodgers was probably looking at a ready to play right-back option due to the World Cup involvement of Mikael Lustig and Cristian Gamboa but declined to offer Odubajo a contract.
While both of those players have returned to action the former Hull defender is still trying to prove his fitness to Brentford.
The defender spent a season at Griffin Park before joining Hull but has still to convince Bees boss Dean Smith that he is worth a new contract.