(John McDonnell/The Washington Post)

Nationals right-hander Doug Fister will be shut down for five days after an MRI taken Friday afternoon in Washington revealed a “minor strain” in his right lat muscle, according to a person familiar with the results.

The Nationals declined comment Friday on Fister’s status. They have yet to place him on the disabled list, a formality that became inevitable once he exited his minor league start Thursday after only one inning. No matter the results of today’s tests, Fister would not have had requisite time to build arm strength for a regular-season start by April 6, the day before he would have been eligible to come off the disabled list. (The Nationals could have backdated Fister’s DL stint to March 23.)

It is not clear how long Fister, whom the Nationals will re-evaluate after his five days without throwing, will remain out. Timetables can vary based on individual circumstances, but ace Stephen Strasburg suffered a minor lat strain last season and returned after the minimum 15 days on the disabled list. Fister’s return will be more complicated, clouded by the need to build sufficient innings for the season.

Fister spent most of March recovering from elbow inflammation, which left him behind schedule. Fister would have thrown 60 pitches Thursday before his lat strain chased him from the start after just 15. Other Nationals starters have thrown at least 80 pitches in their latest outings. Fister has not thrown more than 47 in any one outing. Another five days without throwing will further set him back.

As Fister recovers, both Tanner Roark and Taylor Jordan are expected to open the season in the Nationals rotation. After making impressive cameos last season as rookies, Roark and Jordan had been competing for the final spot in the Nationals’ rotation. The Nationals delayed their decision in case Fister suffered a setback and both would be placed in the rotation.

Roark, 27, went 7-1 with a 1.51 ERA in a late-season call-up last year. Jordan, 25, began last season in Class A Potomac but ascended to the majors on the strength of his vicious sinker. He made nine starts in midsummer, posting a 3.66 ERA before the Nationals shut him down based on their protocol for pitchers in the final stages of recovery from Tommy John surgery.