Managed by Chelsea legend Frank Lampard, Derby knocked out Manchester United in the last round of the competition, with both Mount and Tomori playing a key role in the tie at Old Trafford.
Each has shined since joining the Championship side during the summer, with attacking midfielder Mount earning particularly warm plaudits for his performances.
Loan players are automatically banned from appearing against their parent club in Premier League games, but the Carabao Cup has different rules and it is at the discretion of the parent club to allow one of their own players to play against them.
Explaining the reasoning behind the decision, a Chelsea statement read: “Mount and Tomori have been regulars for the Rams under Frank Lampard this season, and, after beating Manchester United at Old Trafford in the previous round, we believe the experience of facing top-level opposition once more will be of benefit to their on-going development at Derby.”
It may well be something that is criticised should Mount or Tomori play key roles in knocking Chelsea out of the competition, although some would back the longer term thinking and foresight in allowing the pair to not be unnecessarily excluded from a huge game.
Some clubs have been haunted by their own players in the past. Prior to the complete outlawing of the practice in the Premier League, Lomana LuaLua famously scored against Newcastle for loan club Portsmouth after the Magpies allowed him to take part in the game.