Justin Hoyte has signed for Millwall on loan.
The 28-year-old right-back is currently contracted to Boro until the end of the season.
But the switch to Championship rivals Millwall could become permanent when the January transfer window opens.
Boro signed Hoyte from Arsenal for a reported fee of £3m in August 2008.
His initial four-year deal lapsed last year but he eventually agreed a two-year contract on reduced terms before the start of the 2012/13 season.
Signed as a replacement for in-form England international Luke Young, Hoyte never truly established himself at Boro.
He made 31 first team appearances last season but just three, including one start, this term.
At the New Den he'll link up with former Boro team-mates Scott McDonald and Nicky Bailey.