This is one of those risk vs. reward matchups.
If healthy, Max Scherzer versus Rick Porcello is a monster mismatch, far higher than what the lay price is now. But is Scherzer healthy? He only pitched one inning during his last start six days ago because he tweaked his hamstring. 
Indications are he is fine. He had a bullpen session on Sunday and everything went well. I don't think the Nationals would risk Scherzer if they weren't totally convinced he was 100 percent. 
The other hesitation I had was about Washington's hitting. But the Nationals broke out of their hitting slump in a big way on Monday scoring 16 runs against the Mets. Their lineup is healthy again. Juan Soto makes a huge difference. 
Porcello is nothing but a glorified innings-eater. His ERA on the season is 6.92.