Toronto had hoped to gain momentum for a run at an American League wild-card spot in the six games against teams with below .500 records.
Instead, they are 2-3 on the homestand after a 5-4 loss to the Nationals on Tuesday. The teams have split the first two games of the three-game series.
The Blue Jays dropped two of three to the Cleveland Guardians to open the homestand.
The Nationals, meanwhile, assured themselves of a successful nine-game road trip by winning their fifth game win with one left.
Toronto dropped to 3 1/2 games out of the final AL wild-card spot.
Right-hander Chris Bassitt (12-7, 4.00 ERA) is scheduled to start for the Blue Jays on Wednesday. His only two career outings against Washington came last season as a member of the New York Mets, when he went 2-0 and tossed a combined 13 scoreless innings.
Washington is scheduled to start left-hander Patrick Corbin (9-11, 4.70 ERA). He is 1-1 with a 3.75 ERA in two career starts against Toronto, both outings occurring in 2016.
Toronto added shortstop Bo Bichette (strained right quadriceps) to the injured list on Tuesday, a day after third baseman Matt Chapman (finger) landed on the IL.
Bichette opened the month on the IL with a knee injury. He left the Sunday game due to quad tightness.
"He's frustrated," Blue Jays manager John Schneider said. "At the same time, he understands that he doesn't want to do anything that's going to put him out for the season. He took it as well as he could is the best way to say it. He wants to be out there every day, especially at this time of year."
Ernie Clement, recalled from Triple-A Buffalo on Monday, started at shortstop the past two days.
Toronto selected the contract of infielder Mason McCoy on Tuesday. He was acquired from the Seattle Mariners for reliever Trent Thornton in a July 26 trade. McCoy, who has no major league experience, hit .226 with 12 homers and 64 RBIs in 108 Triple-A games this year.
"He's a really, really good defender and great baserunner," Schneider said. "He's kind of another blue-collar dude who's been working his entire career to get here. I'm excited to have him."
Davis Schneider continued his hot hitting since his promotion from Buffalo with a solo homer and an RBI double Tuesday. In his first 13 major league games, he is batting 19-for-45 (.422) with four doubles, six homers and 14 RBIs for the Blue Jays.
Toronto's Vladimir Guerrero Jr. extended his hitting streak to 10 games with a first-inning single. He is 13-for-43 (.302) over that stretch.
The Nationals got a three-run homer from catcher Keibert Ruiz and a two-run shot from Carter Kieboom.
It was a special night for Washington center fielder Jacob Young, who bunted for his first major league hit, had his first stolen base and threw out Alejandro Kirk at home for his first assist after catching a fly ball in the eighth.
"There's no one else on base, so you can kind of let it fly," Young said. "You want to give Keibert a chance, and Keibert made a heck of a play."
In making the catch, Young nearly ran into left fielder Alex Call but called him off in time.
"We had that conversation the first day that he got here," Nationals manager Dave Martinez said. "'You're the center fielder. You control the outfield. You call it. You take everything.'"
—Field Level Media