Marcel Louis-Jacques of ESPN noted Bills head coach Sean McDermott said the condition was discovered during an NFL-mandated checkup as Sweeney neared a return from the physically unable to perform list with a foot injury.
According to Louis-Jacques, the Bills put Sweeney on the reserve/COVID-19 list in Week 7 since he was considered a close contact to fellow tight end Dawson Knox, who tested positive for the virus.
Buffalo selected the Boston College product with a seventh-round pick in last year's NFL draft, and he appeared in six games as a rookie.
He has not played this year because of the injury and his time on the reserve/COVID-19 list.
As Louis-Jacques pointed out, Sweeney is the first known NFL player to be diagnosed with myocarditis this season as play continues amid the COVID-19 pandemic. Notably, Boston Red Sox pitcher Eduardo Rodriguez missed the 2020 campaign after being diagnosed with myocarditis.
"That's the most important part of your body, so when you hear that, the first time I hear it was kind of scared a little,'' Rodriguez said in July, per Kyle Hightower of the Associated Press. "Now that I know what it is it's still scary, but I know exactly what it is. Just talk to my mom, talk to my wife, they know what I have and everything. Now we just gotta take the rest. That's hard, but you gotta take a rest.''
Myocarditis was also one of the primary concerns when the Big Ten elected to postpone its football season before eventually returning in October.
As for the Bills, they are scheduled to face the Los Angeles Chargers on Sunday.