Williams, who made clear his desire to leave Glanford Park on numerous occasions before his exit to Reading at the end of the last campaign, has been out of favour at the Madejski Stadium this season.
United are hoping to strengthen the left back position in the January transfer window- with a permanent move for Eddie Nolan still in the balance despite looking increasingly unlikely.
"Marcus's wages have probably rocketed out of our bracket. He left the club and wanted to seek his future elsewhere. I don't think he'd be an option for us," Ian Baraclough, the Iron manager, told BBC Radio Humberside: