This is a low price to get an elite starter versus a No. 3 type pitcher with Max Scherzer facing Zach Wheeler.
Scherzer may be past his prime, but he's still a dominant Tier 1 pitcher. He has a 3.86 ERA after a bad stretch. The buy sign is back on him, though, after his last performance. Scherzer gave up one run on six hits with 11 strikeouts and no walks in six innings against the Red Sox this past Friday. Scherzer said he tweaked his mechanics and that made a difference. 
Scherzer is 11-3 with a 2.62 ERA in 20 career starts against the Phillies. This includes a 5-0 mark with a 2.40 ERA at hitter-friendly Citizens Bank Park. 
This isn't a huge fade on Wheeler, who has pitched well this season. He's just not in the class of Scherzer and the Phillies have a worse bullpen than the Nationals. Wheeler also has a bad history versus the Nationals with a 5-10 record and 5.01 ERA in 18 starts.