Johnstone in the final nine months of his contract with Championship side West Brom and is a target for clubs in the Premier League.
Tottenham and West Ham have identified the England international as a potential recruit, while the Baggies are prepared to hand him a lucrative deal to make him stay.
Southampton, who have identified the goalkeeping department as an area they need to strengthen next year, are also interested in signing Johnstone.
The Saints are also prepared to pay West Brom a transfer fee to sign him in January rather than wait until he becomes a free agent in the summer.
Tottenham and West Ham are claimed to have pencilled in offers for the goalkeeper if he leaves and Southampton could look to fend off competition by offering West Brom a transfer fee.
Johnstone, though, appears to be ready to see out his contract before deciding on his next destination.
While the Englishman is prepared to wait until his contract comes to an end, it remains to be seen if Southampton can convince him to move in January.