Celtic could turn their attention to Scotland goalkeeper Craig Gordon after missing out on Poland international Gregorz Sandomierski.
It is believed that there is interest from the Scottish Premier League club in the 28-year-old, although a deal is still a long way off.
Gordon is close to a return after undergoing knee surgery in April.
However, his position is in doubt after Sunderland recruited Kieran Westwood from Coventry City in the summer.
Westwood was on the bench when the Black Cats opened the Premier League season in England on Saturday with a 1-1 draw away to Liverpool, with Simon Mignolet starting in goal.
Gordon joined Sunderland from Hearts for £9m in 2007, but a previous knee injury and a broken arm have hampered his time at the Stadium of Light.
Celtic are seeking cover for the number one jersey, with Lukasz Zaluska's form raising questions over his long-term future as the club's first-chioce keeper.
Fraser Forster enjoyed a successful loan spell at the Glasgow club last season but returned to Newcastle United for the start of the current term.
Croatian goalkeeper Stipe Pletikosa had been on trial at Celtic Park but has joined Russian top-flight outfit Rostov from Spartak Moscow after spending last season on loan to Tottenham Hotspur.
Despite bids from Celtic for Poland goalkeeper Gregorz Sandomierski, the 21-year-old last week opted to join Belgian champions Genk instead.