Republic of Ireland midfielder Josh Cullen, manager David Moyes and Issa Diop will all have to isolate for the next week and a half after the West Ham trio tested positive for Covid-19.
Cullen and Diop had been named in the Hammers team to face Hull last night, while Moyes had been at the London Stadium for almost an hour before they were all sent home.
The hosts had their first 11 ready to go for the Carabao Cup match, but 15 minutes later the line-up was changed with the two players omitted, and when the teams came out there was no sign of Moyes, with assistant Alan Irvine taking charge.
Irvine said: "At 20 past six we suddenly got some information from our doctor and head physio to say there were three positive tests, the manager and the two players.
"We'd had contact in so far as we were in the same area of the stadium as David.
"We'd been at the stadium for about two hours before the match started.
"We were shocked by the news, we've had so many tests and not had any positive.
"But we've been very particular about protocol right from the start, the medical team are very aware of what has been happening and we've followed all the protocol, and we'll be following protocol now."
West Ham revealed all three are asymptomatic and will now follow Public Health England and Premier League guidelines and protocols, which will mean they have to isolate for 10 days.
It is understood West Ham's and Hull's medical departments agreed the match could go ahead within the protocols and in consultation with the EFL's medical department, and given the evidence that suggests there is very low risk of transmission during a match outdoors.
However, with the news coming on the same day that nearby Leyton Orient had their game against Tottenham postponed due to a number of positive tests, and just as UK Prime Minister Boris Johnson was outlining new coronavirus restrictions including no prospect of fans returning to matches any time soon, it was not a good look.
Nevertheless, Hull said in a statement: "We are satisfied that West Ham followed all guidelines and protocols upon receiving the positive results and, after consulting with the EFL, were comfortable with the game going ahead."
City boss Grant McCann added: "We found out just before the team sheets were going to go in, probably an hour before the game.
"We did not come into contact with Moyes or the others. We are happy we followed the EFL protocols and West Ham were adamant on that so we were comfortable to play the game.
"The players knew before the game. The players are not silly. They all knew about it. Once we got here we had to follow the guidelines."
West Ham's next match is at home to Wolves on Sunday, while it is understood Arsenal, against whom they played at the weekend, will continue as planned as all protocols were followed.