Two days before the Minnesota Vikings face one of the best teams in the N.F.L., quarterback Kirk Cousins tested positive for the coronavirus and was placed on the team’s Covid-19 reserve list, a move that will sideline him for at least five days.

Cousins and the Vikings (7-8) are one game back in the N.F.C. playoff race, a gap the team had hoped to close Sunday night in prime time against the Green Bay Packers. Instead, Cousins’s absence will deal a serious blow to his team’s chances, a familiar scenario in a season in which rampant positive tests among N.F.L. players and coaches has made chaos of teams’ preparations.

After more than 90 N.F.L. players tested positive on Monday, the N.F.L. and the N.F.L. Players Association agreed on Tuesday to amended this season’s Covid-19 protocols to mirror new guidance from the Centers for Disease Control and Prevention. Vaccinated and unvaccinated players can return from quarantine in five days, not 10, if they are asymptomatic. The league has a 94 percent vaccination rate among players.

The change has given other teams attempting to make the playoffs hope that star players will be able to return before important games on Sunday and Monday, the penultimate week of the regular season. Indianapolis Colts quarterback Carson Wentz, for example, was among 39 players who tested positive on Tuesday. He will be evaluated Sunday before his team’s game against the Las Vegas Raiders, who are two games back in the A.F.C. playoff race.