The Blues have taken the unusual step to give permission for the duo to play against their parent side in the Carabao Cup tie at Stamford Bridge.
Both men have been instrumental on loan at the Rams this season under the tutelage of Chelsea legend Frank Lampard.
Between the two players, they have racked up 29 appearances in all competitions – establishing themselves as key players in Derby's starting XI.
A club statement from the West Londoners said: "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."
While a grateful Lampard said: "I would like to take this opportunity to thank Chelsea, their director Marina Granovskaia and manager Maurizio Sarri for allowing Mason and Fikayo to be available for selection for the Carabao Cup tie.
"We’ve been in dialogue with Chelsea since the draw was made and it’s great, first and foremost, that the players will have the opportunity to test themselves against a top side."
One Blues fan tweeted: "I actually disagree with this. I agree that Both Mount and Tomori will benefit in terms of development.
"However, it’s about the here and now and Chelsea’s progression in the cup. I’m not saying this is going to mean we’ll get beat, but it certainly gives Derby a better chance!"
While another commented: "Scenes when Mount scores against us and we go out."
Another added: "I could understand the decision if you chose not to, but I can appreciate the gesture, just hope it doesn’t come back to bite us."
One supporter was behind the move by Chelsea, as he stated: "Best bit of decision the club have made in recent times."
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