Matthew Bates is training with Bristol City, the Championship club have confirmed.
The 25-year-old has recently returned to full training following a fifth cruciate knee ligament operation.
Bates was training with Middlesbrough, but had been out of contract since the summer.
He held talks with Boro over a new deal, but they could not agree terms so he moved on.
Now he has linked up with Bristol City, and their boss Derek McInnes said: "He's left Middlesbrough and is pleased to be training with us.
"Obviously he needs to build his fitness up after his latest injury, and we'll be having a look at him for a week or so."
Sky Sports also understands that a host of Premier League and Championship clubs are monitoring Bates's situation.
Southampton, Swansea, Wigan, West Ham, Leeds, Bolton and Hull City are all thought to be considering a move for Bates - who can play in defence or midfield.