Middlesbrough and Sheffield Wednesday's preparations for Saturday's FA Cup third-round ties have been affected by positive Covid-19 cases within first-team players and staff.
Boro are scheduled to visit Championship rivals Brentford, while the Owls are away at League Two side Exeter City.
Both clubs have had to close their training grounds.
Derby confirmed a similar outbreak before their FA Cup tie at Chorley.
The Rams have isolated their entire first-team squad and first-team coaches, including interim boss Wayne Rooney, for the game and will instead field a side comprising of under-23 and under-18 players.