Tony Pulis told BBC Radio Stoke : "I've spoken to Carlisle and he will stay there, he's getting good experience and that's what we want.
"We want younger players to get out as early as they can to play against men."
Both Marshall and Matty Lund joined from Crewe 18 months ago after being recommended by then academy director Steve Holland, a former youth coach and first-team manager at Gresty Road.
Lund is currently out on loan at League Two club Hereford United.
American trialist Teal Bunbury is still waiting to hear if he is to have a long-term stay with the Potters.
The 20-year-old USA international striker scored in a reserve game against Wigan Athletic but it may not be enough to earn him a deal with the Premier League outfit.
"Whether he's what we want for the next step, we'll have to see," Pulis added
"But he's a nice kid and, like most American sports people, is very dedicated. His attitude during his time here has been first-class."
Source: BBC Sport
Source: BBC Sport