Harper left the game in the fourth inning with an injury, the team said.
And Mike Rizzo and Matt Williams explain why only one pitcher was called up on Sept. 1.
Roark made an abbreviated start on Monday in the minors as insurance for Strasburg's back injury.
Rizzo said Williams pushed all the right buttons to put Nats in a position to succeed.
COLUMN | Shared despair is the perfect bonding agent for followers of Nationals and Redskins.
The Nats manager is committing these three egregious errors.
Discuss here: Max Scherzer vs. Tyler Lyons.
The Nationals will use the same lineup again on Wednesday, the series finale in St. Louis.
Harper said: "If I get on first base, I’m doing my job. That’s what I'll keep doing."
From Joe Ross to Yunel Escobar to Drew Storen to Matt Williams, many played a part in Tuesday's deflating loss.
In order to make room for Severino, the Nationals transferred reliever David Carpenter to the 60-day disabled list.
Tanner Roark had a shortened start in the minors on Monday so that he could potentially start in Strasburg's place on Friday.
Discuss here: Joe Ross vs. Marco Gonzales.