Stephen Strasburg‘s return to dominance will have to wait, stalled by another out-of-the-blue injury in the midst of what his teammates thought was his most impressive outing of the season. The Nationals‘ right-hander left Saturday’s game in the fourth inning after feeling tightness in his left side. Sunday afternoon, the Nationals placed him on the 15-day disabled list with a strained left oblique, his second disabled list stint in two months.

Nationals Manager Matt Williams said Strasburg underwent an MRI exam that revealed the strain, which would “certainly not” have let him make his next start in turn and therefore required the shutdown.

“It’s unfortunate because he felt good and was pitching great,” Williams said. “As you go through games and things, you can feel something and it can go away and it never comes back. But that one didn’t go away.”

The 26-year-old has battled injuries all season, struggling with strains to muscles in his back and side that prevented him from pitching comfortably. His stuff — 97-mph fastball, etc. — never abandoned him, though at times his results were poor. Before landing on the disabled list on the last weekend of May with what was officially called a strained left trapezius muscle, Strasburg lasted into the fifth inning once in five starts.

Then came that disabled list stint, which allowed him to recover his health and his mechanics. He returned with five strong, scoreless innings against the Braves on June 23, then pitched seven dominant innings in which he allowed two runs and struck out nine. On Saturday, Strasburg was “carving,” as Bryce Harper put it, and pitched into that fourth inning with noticeable command and body language that suggested total comfort.

Then came the pain in his side, which Strasburg said he had never felt before the fourth-inning pitch to Buster Posey on which it emerged.

“There’s a difference between him not being able to do things when he’s out on the field because something’s bothering him, and this,” Williams said. “This is something he just can’t pitch with. That’s the difference that comes to my mind, anyway.”

Oblique injuries are notoriously unpredictable, and “everybody’s different,” said Williams, who therefore would not establish a timetable for Strasburg’s return.

“We know it’s 15 [days]. We’ll do what we can in those 15 and see where we’re at,” Williams said. “We went through it last time he went on the DL and having to make sure he’s fully ready. So we’ll see. It reacts differently with everybody, and 15 days from now we’ll have a  better answer, I know that.”

Tanner Roark relieved Strasburg on Saturday and seems likely to slide into his place in the rotation — though the Nationals do not need a fifth starter until after the all-star break. Roark won 15 games last season and has pitched well in the six starts he’s made for Washington this season. The Nationals recalled right-hander Taylor Jordan from Class AAA Syracuse to take Strasburg’s spot. Jordan will pitch in long relief out of the bullpen for now, as Roark was filling that role. Williams did not commit to Roark rejoining the rotation, committing only to the scheduled starters for this week’s series against the Reds — Doug Fister, Max Scherzer and Gio Gonzalez — and nothing more.