Northampton Saints' match against Leicester Tigers is the fifth Premiership fixture to be cancelled due to Covid-19
Diamond’s shock departure comes just three games into the new Premiership season.
A third opponent will be announced for 2021 in due course.
Andy Farrell’s first year in the job was assessed as “an average return” by the IRFU.
The Leeds director of rugby is already well past his fundraising target of £77,777.
The duo are struggling with groin problems ahead of the Autumn Nations Cup third-place play-off.
Farrell’s tenure has come under scrutiny due to a series of inconsistent performances and results.
The Irish failed to impress as they set up a third-place play-off with Scotland in the Autumn Nations Cup.