Moore has only been back for one game since contracting Covid-19, and his latest illness is linked to that.
He took the team for Tuesday's 2-0 defeat to Cardiff City and told reporters afterwards he was feeling much better, but was evidently putting a brave face on things.
“I have developed pneumonia as a result of Covid-19, with blood clots on the lungs and extreme discomfort,” he told his club's official website.
“I am gutted to be away but I understand the situation and the course of action I must take.”
Moore will now stay away from the club again just as his first-team coach Paul Williams returns from a 10-day period of isolation having contracted coronavirus two days after Moore did.
Assistant manager Jamie Smith is set to take the team on Saturday, as he did for the first three matches after March's international break when Moore was in isolation. In doing so, Smith became the sixth person to take charge of the Owls on a matchday.
Garry Monk and Tony Pulis had spells as manager earlier in the season. Under-23 manager Neil Thompson took the reins after Pulis's sacking in December, but was forced to miss the FA Cup third-round tie at Exeter City when he also contracted Covid. Academy coach Andy Holdsworth led the team that day.
Moore's health situation is all the more concerning because he is asthmatic.
“It's been something I've had to manage all my career," he explained last week. "There's only been one time when I'd have to pull out of a game because they'd got the grass on the morning and it was a hot afternoon and I couldn't continue because I was short of breath but overall it's something I've handled quite well with the inhalers and keeping on top of it.
“You manage it and that's why with this illness (Covid-19) we know how serious it is but I must say the doctor Richard was very good and hes's not really left me, he's been on it."
Before Thursday's game between Rotherham United and Coventry City, the Owls were eight points from Championship safety with five matches to play.
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