Republic of Ireland international Josh Cullen has tested positive for COVID-19, alongside West Ham teammate Issa Diop and Hammers manager David Moyes.
Cullen and Diop were initially named to start in West Ham's 5-1 win over Hull City in the Carabao Cup on Tuesday night.
Both players, and Moyes, were present at the match, before receiving their test results before kickoff at the London Stadium.
"West Ham United can confirm that David Moyes, Issa Diop and Josh Cullen have returned positive tests for COVID-19," a club statement read.
"All three, who are asymptomatic, will now follow Public Health England and Premier League guidelines and protocols.
"Everyone at West Ham United send their best wishes to David, Issa and Josh."
The trio will now miss the Hammers' home match against Wolves on Sunday, as well as the fourth round of the Carabao Cup.
Hull were made aware of the three positive tests before the match, but both sides were happy to proceed with the match.
"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," Hull said in a statement.
Moyes' assistant manager Alan Irvine then took charge of the team, who secured a resounding victory through goals from Sebastien Haller (two), Andriy Yarmolenko (two) and Robert Snodgrass.
Cullen, 24, has returned to the Hammers for the 2020/21 campaign after impressing on loan at Charlton for the past two seasons.
The combative midfielder has captained the Boys in Green at U21 level and has made two appearances at senior level.
Elsewhere in the Carabao Cup, Leyton Orient's clash with Spurs was postponed due to positive tests.
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