English football was reeling by coronavirus outbreaks, including one at Manchester City which forced their Premier League game at Everton to be postponed.
Three days after Kyle Walker and Gabriel Jesus tested positive, City announced another round of testing had "returned a number of positive cases".
The news came as 12 games in England's lower tiers were postponed due to COVID-19 on Monday.
Furthermore, Arsenal defender Gabriel tested positive for coronavirus, ruling the Brazilian out of the Gunners' matches against Brighton & Hove Albion and West Bromwich Albion.
City lodged a request to postpone their match at Goodison Park on Monday morning and confirmation came about four hours before kick-off after a hastily-arranged Premier League board meeting.
The Premier League, in agreeing to the postponement, insisted the health of players and staff was paramount.
"The board agreed to rearrange the game as a precaution, and further testing will now take place tomorrow," said a statement.
"The decision has been taken with the health of players and staff the priority."
Second-placed Everton, who could have gone level on points with Liverpool at the top of the table with a victory, had no say in the decision to postpone the match and said they would ask the Premier League for a full explanation of the decision-making process.
The latest COVID-19 clusters left Chelsea manager Frank Lampard in the dark over his side's next two games - a league clash with City on Sunday and their FA Cup third-round tie with fourth-tier Morecambe, who have their own virus concerns.
In the NFL, the Cleveland Browns' hopes of ending a play-off drought stretching to 2002 with a win over the Pittsburgh Steelers could be boosted by the return of key players who missed their loss on Sunday because of COVID-19 protocols.
Six players, including the Browns' top four wide receivers, will be able to return to action this week as long as they test negative for the virus.
The news was not as positive for the Baltimore Ravens, however, who were fined $250,000 ($A330,000) by the NFL for violating COVID-19 protocols.
The violations led to a coronavirus outbreak, which involved 23 players and several support staff, and the rescheduling of the Ravens' Thanksgiving night game against Pittsburgh.
"We respect the league's process and discipline process, and we accepted the discipline," said Ravens coach John Harbaugh, who was wearing a mask during his weekly Zoom interview the media.