If a golfer playing in the Ryder Cup later this week tests positive for COVID-19, a contingency plan is set, following a rarely-used rule involving secret names placed in envelopes.

European captain Padraig Harrington said on Tuesday that he and U.S. captain Steve Stricker agreed to have a "COVID envelope" in addition to an injury envelope - to be used in the event that the coronavirus takes one of their players out of competition.

Harrington and Stricker will place the names of three of their players into an envelope before Sunday's 12 singles matches at Whistling Straits in Kohler, Wisconsin. If one team has someone forced out by COVID-19, the other team would then sideline the top player from the COVID envelope list to re-balance the matches, and the point would be halved.

"There's an injury envelope and there's a COVID envelope, so that's two separate envelopes at this stage," Harrington said. "Maybe the same name is in both envelopes. That's as far as I'm aware at this moment."

RELATED - U.S. eyes Ryder Cup win without Tiger Woods and Phil Mickelson

Harrington said the teams haven't clarified "how many is too many" to call off the match in the event of a COVID outbreak among one team.

"You have to be prepared for these things, and as I said, (the envelope system) has been there all along, so it's nothing really new," Harrington said.

"It's obviously highlighted because of COVID, but there's always been a name in the envelope for an injury, just like coming into this we were asked to be aware that having somebody as a reserve, somebody as a backup who you would bring along, and what happens if somebody pulls out because of COVID."

Once the competition begins on Friday, neither team will be able to add any alternate players.

The envelope contingency is only necessary for the singles matches that conclude the three-day event. During four-ball and foursome matches on Friday and Saturday, only eight players are competing at a time for either team, so if someone were to be sidelined by illness or injury, the captain would choose one of his remaining available golfers to serve as a substitute.