Cougars requested the League 1 game to be called off under the sport's Covid-19 protocols.
An RFL statement read: "After one member of Keighley’s senior squad tested positive for Covid-19 last week, a second senior player was confirmed positive this morning.
"That took the number of other players required to isolate as close contacts to nine, meaning the Cougars meet the threshold of a minimum of seven players ruled out as a result of Covid-19 - allowing them to request the postponement.
"The clubs will now liaise with the RFL on a possible rearrangement of the match later in the summer."
It is the first League 1 game to be postponed this season due to the pandemic but follows two Super League games being called off last week and also tomorrow's contest between Catalans Dragons and Leeds Rhinos.