That potentially further weakens a lineup entering Sunday's pivotal game against the Philadelphia Eagles (8-7). Washington (6-9) remains a playoff long shot but could stay in the race by beating the Eagles.
Both Gibson and Flowers are vaccinated and could "test out" of isolation sooner than five days, as soon as they test negative -- either with two tests taken at the same time, or in combination with a cycle threshold (CT) test reading that indicates they are no longer contagious.
Starting right tackle Sam Cosmi and punter Tress Way also are on the list, as is key backup cornerback Darryl Roberts. Starting corner William Jackson III was ruled out after not practicing this week because of a calf injury.
Defensive end Montez Sweat is questionable after missing practice this week following the death of his brother Tuesday night.
Gibson's availability already was in question because of a toe injury that caused him to miss Thursday's practice. If he doesn't play, undrafted rookie free agent Jaret Patterson, who has rushed for 188 yards on 50 carries this season, would get the start.
"I'm ready," Patterson said Thursday. "My mindset has always been like that, just staying ready for whatever opportunity comes my way."
Gibson, who has played in every game, has rushed for 891 yards and six touchdowns this season. Saahdiq Charles, who started two games at right guard in place of Brandon Scherff, would replace Flowers.
Coach Ron Rivera said cornerback depth is "very thin right now." Starting corner Kendall Fuller missed practice Friday for personal reasons.
"I'm hoping we can keep everybody healthy through this weekend and get those guys back next week," Rivera said. "It is precarious. We just continue to go through this and it's just one of those things that's lingering right now."
Washington already has nine players on injured reserve who either were starters or played substantial snaps, including tight end Logan Thomas, running back J.D. McKissic, quarterback Ryan Fitzpatrick, safety/linebacker Landon Collins and defensive end Chase Young. The WFT also is starting its fourth center.
Washington has lost three consecutive games, with 23 players having been on the reserve/COVID-19 list at one point. Eight remain on the list, and more than half the roster has been on there at some point. Eight assistant coaches have been on the list, in addition to multiple staffers throughout the organization.
"You kind of worry about what's happening right now," Rivera said when asked if it seems like it never ends. "We're not the only team it's happening to. A few others are going through it right now. It seems to be inevitable for everybody too. It drags on."