Collier added Strasburg will undergo an MRI.
Per MASN's Dan Kolko, Strasburg appeared to be favoring his right shoulder after throwing pitches. He allowed three runs on five hits with two strikeouts and two innings before being taken out.
It's the first setback of the 2018 season for Strasburg, who avoided major injuries during a prosperous 2017 season.
In 28 starts a year ago, the three-time All-Star went 15-4 with a career-best 2.52 ERA, 1.015 WHIP, 204 strikeouts and 47 walks.
Now 12 starts into his ninth MLB season prior to Friday, the 29-year-old has posted a record of 6-5 with a 3.20 ERA, 1.06 WHIP and 93 strikeouts compared to 19 walks in 78.2 innings.
The Nationals can survive a short-term absence from Strasburg, but anything beyond that could prove detrimental to a pitching staff that has ranked among the league's best when he's been at the top of his game.
In the meantime, the onus will be on Max Scherzer and Gio Gonzalez to hold down the fort at the top of the rotation.