Doug Bergqvist has returned to Farnborough on loan.
It is the second time that Boro manager Spencer Day has signed the defender this season.
The 18-year-old Swede made over 30 appearances for Boro in his previous spell with them, which expired in January.
He has now returned to Cherrywood Road on a deal running until March 31, although this may reportedly, be extended at a later date.