According to the Chronicle Live, a Sunderland area newspaper, they have a article saying -

Sunderlands Rotherham United friendly fixture is to go ahead, despite the Millers injury woes, injuries have forced the Millers game against Grimsby Town to be cancelled.
The Millers cancelling the game so they can work on the injuries of the players for the Sunderland visit.

Why don't they put the truth, Rotherham haven't enough players in the squad. to cover the injured players.