The Nationals placed right fielder Bryce Harper on the 10-day disabled list with a "significant bone bruise" in his left knee, the team announced Sunday.
The Blue Jays catcher was seen grimacing during his first inning at-bat after he appeared to slip running out of the batter's box on a ground out to third base.
Mercer added a solo homer off Leonel Campos in the ninth.
Happ (6-8) allowed one run and four hits, walked three and struck out eight, improving to 3-0 with a 1.45 ERA in August.
Toronto lost for the first time in seven interleague games.
With one on the first, Donaldson put Kuhl's fastball into the second deck in left for his 15th home run of the season.
In five games with the Blue Jays this year, Ohlman is 2-for-9 (.222) with an RBI.
As the Jays defeated the Pirates on Sunday, manager John Gibbons recorded his 700th career victory with the franchise, second most behind Cito Gaston, who won 894 games as Blue Jays skipper.
Juan Nicasio pitched around a single in the eighth for Pittsburgh (58-58).