FC United manager Karl Marginson was admitted to hospital hours before their pre-season game on Tuesday night.
The 40-year-old boss was not on the touchline for the Rebels’ trip to Trafford after complaining of severe abdominal pains.
He stayed in hospital overnight and was set to undergo additional tests yesterday in a bid to diagnose the problem.
Assistant boss Roy Soule took over managerial duties for the 2-1 defeat at Trafford – the final friendly before their league campaigns start this weekend.
Marginson is expected to be absent for Saturday's Evo-Stik Premier opener at Stafford Rangers, with Soule again taking charge.