Whites head coach Marcelo Bielsa has revealed that January recruit Casilla will take the place of Peacock-Farrell having played for United's under-23s against Southampton on Monday night.
Bielsa has again already named his entire starting XI for Saturday's clash against the Millers for which Jansson serves a one-game ban for being sent off following a second booking in last weekend's 2-1 loss at Stoke City.
Phillips, who himself has now completed a three-match ban for his dismissal in the 4-2 loss at Nottingham Forest, will replace Jansson at the heart of the Leeds defence.
Asked if Casilla would start in goal against the Millers, Bielsa said: "Yes. The team will be Casilla, Ayling, Phillips, Cooper, Alioski, Forshaw, Klich, Hernandez, Clarke, Roofe and Harrison."