Mikael Silvestre is training at Manchester United, four years after leaving Old Trafford and at a time when Sir Alex Ferguson is wrestling with a defensive injury crisis.
The France international made 361 appearances in a nine-year United career, winning five Premier League titles before joining Arsenal in 2008.
Now without a club after leaving Werder Bremen in the summer, he has been allowed to work at his former club as he seeks to regain sharpness. As a free agent, he could be signed outside the transfer window.
Silvestre had a trial with West Ham without earning a contract at Upton Park, and the presence of the 35-year-old at United's Carrington ground has prompted speculation that Ferguson will offer him another contract.
The Scot is without three of his senior central defenders, with Nemanja Vidic, Phil Jones and Chris Smalling all sidelined, leaving only the injury-prone Jonny Evans and Rio Ferdinand available.
Ferguson gave debuts to two central defenders, Scott Wootton and Michael Keane, in last week's Capital One Cup victory over Newcastle, and included both in his squad for the Champions League clash with Cluj.
A former Rennes player, Silvestre joined United from Internazionale in 1999 and was a regular in his first seven seasons at the club.
As an unused substitute, he was a Champions League winner in 2008 before becoming the first player since Brian Kidd to go direct from United to Arsenal.
However, he had an unsuccessful stay at the Emirates Stadium and has only made 69 league appearances since 2006.