Even though he has blasted two home runs, one double, six runs batted in, eight runs scored and a.320/.387/.600 slash line, the outfielder has only started on six of his 14 appearances in 2017, including Friday's game.
New York Mets left fielder Yoenis Cespedes will undergo an MRI on his left hamstring and manager Terry Collins called the injury "a concern".
1B Lucas Duda (hyperextended left elbow) did not play Thursday night, when the Mets fell to the Phillies 6-4. Flores, meanwhile, is dealing with a knee infection that will keep him in the hospital throughout the weekend. Rivera and Sean Gilmartin taking their places on the 25-man roster.
Cespedes limped out of Thursday night's loss to Philadelphia in the fifth inning.More news: Rockets overcome Westbrook's 51, take 2-0 lead over Thunder
Collins said it "has not been determined" how many days Cespedes would miss.
"We have Monday off, but I think unless he makes a miraculous recovery today, we'd be foolish to push him with the extra day on Monday", Collins said.
Jared Diamond of the Wall Street Journal noted the Mets are saying Cespedes has a cramp in his left hamstring.
Duda is hitting.238 with four homers and seven RBIs.More news: Serena Williams expecting baby this year
The Nationals may also be at less than 100 percent for at least the start of the series. It is the first trip to the majors this season for Plawecki, who will provide the Mets some depth at catcher and an extra bat off the bench as starting C Travis d'Arnaud recovers from a right wrist contusion. Shortstop Trea Turner, who is on the 10-day disabled list with a hamstring injury, is expected to rejoin the club Friday, though he may not be activated until Saturday.
DeGrom doesn't have a decision in three starts this season despite a 1.89 ERA.
Flores is a career.254 hitter in five seasons with the Mets.More news: UK prime minister defends decision to seek snap election