From the PL
In the Premier League and FA Cup, a loan player cannot be fielded against his parent club.
The Football Association's rule for men's FA Cup football is that a loan player "cannot play in a competition match against their parent club", while the Premier League handbook states that "during the period of the temporary transfer... a player shall not play against the transfer club".
Players can face their parent club in the League Cup, according to English Football League rules, but that club must give written consent.