The centre-back, who recently signed a new contract with the club, will link up with Paul Simpson and his side as they look to move up the table in League One.
Grehan has been in strong form for Darren Powell’s Under-21s side this season, with 17 appearances, two goals and an assist to his name. He has also been captain of the side this season.
His performances for the Under-21s have also seen him train with the first-team squad on multiple occasions and earn a place in the matchday squad for the Carabao Cup second round tie against Plymouth Argyle.
Additionally, Luke Plange will return to the club following the conclusion of his loan to Carlisle.