Blues are in the market for a new number one after Joe Hart's stellar season at St Andrew's on loan from Manchester City.
Manager Alex McLeish had already accepted that Hart would return to Eastlands this summer with no chance of a permanent transfer.
He is now understood to have turned to Foster, who is available after falling down the pecking order at Old Trafford.
The 27-year-old made 12 starts for United in all competitions this season, but has not featured since a 3-0 Premier League home win over West Ham in February.
Foster won his fourth England cap in last October's 1-0 friendly defeat to Brazil in Qatar but will almost certainly miss out on the World Cup.
He began his professional career with Stoke before moving to United in a £1million deal in 2005.
He enjoyed two hugely successful loan spells at Watford in the 2005/6 and 2006/7 seasons but has found opportunities severely limited at United in the last three years due to the form of Edwin van der Sar and Tomas Kuszczak.