Surely he can't play. Perhaps you're thinking of the transfer window, instead of the deadline?
The window, which only applies to Premiership clubs, allows you to sign Premiership players (or Prem clubs to sign other players) if they are without a contract ot without a club, even if it's outside the window.
The transfer deadline, however, applies to all clubs, and means no team can play someone they have signed after that date. It's to stop clubs signing a superstar on loan just to ensure they stay up, or win promotion, or win the cup etc.
BC's website might say it's a boost, but they're probably talking in psychological terms rather than in terms of a player who will help them out on Sunday.