Manchester United have identified Massimiliano Allegri as a potential replacement for under-fire manager Ole Gunnar Solskjaer.
Solskjaer guided the Red Devils to an impressive third place finish last term after they went unbeaten following the project restart.
However, things have been different this season and they have already suffered two defeats from the opening three league games.
Meanwhile, they were also lucky to come up with a 3-2 win at Brighton. The Seagulls managed to hit the woodwork on a record five occasions.
Mauricio Pochettino is obviously the leading option to succeed Solskjaer at the helm, but Castles claims that Allegri is another one under consideration for the head coach role, suppose it becomes vacant.
The Red Devils are scheduled to face Newcastle United on the road when they return from the international break this weekend. They have struggled at St.James Park of late with just one win from the previous four meetings.