The pair are expected to play in the PDL fixture against Millwall at the St. Andrew's Trillion Trophy Stadium on Saturday (1pm).
The move is designed to help them build up their match fitness and sharpness after injury.
Roberts injured his calf at Blackburn Rovers in December. He rejoined full training last week and was on the bench for the Aston Villa game.
Lakin has been on the mend from a thigh injury and also rejoined the main group last week.
Manager Garry Monk said: “Robbo was on the bench (against Villa) purely because we only had 17 players.
"Robbo and Charlie had just come back into training, so it was to put a position on the bench that might be used.
"They are not fully fit yet. They’ll be playing in the game on Saturday to try and get some minutes with the Under-23s.
"We’ve got another game during the international break where those two especially will need minutes.
"I expect them to be more fully fit and up for selection when we come out of the international break.”
Blues Under-23s go to QPR a week on Friday, 22 March in the PDL (2pm).
Admission to Saturday's game is free for Season Ticket holders and Blues Loyalty members. Otherwise it is £4 and £2.
Blues (from): Weaver, Hutton, Forrest, Burke, Bajrami, Roberts, Stirk, Lakin, Boyd-Munce, O'Keeffe, McCoy, Bellingham, Bailey, Hurst.