Stephen Strasburg is relieved after 16 pitches in the second inning oagainst the Cincinnati Reds Friday. (AP Photo/John Minchillo)

CINCINNATI — The Nationals officially placed right-handed starter Stephen Strasburg on the 15-day disabled list Saturday with “neck tightness.” Nationals Manager Matt Williams said the pitcher had “back issues.” The reason for the ambiguity is simple: the Nationals do not know what is plaguing Strasburg. He is going on the disabled list so they can find out.

Williams said the team has not determined who will start in Strasburg’s place next week, but the Nationals recalled right-hander Taylor Hill from Syracuse to take Strasburg’s place on the roster and pitch long relief.

“We gotta get to the bottom of what’s going on,” Williams said. “It’s perplexing because it’s not anything you can point to. So Stras will go on the DL, get this thing worked out, and hopefully be back in 15 days.”

“This thing” that Strasburg must get worked out has contributed to the worst stretch of the 26-year-old’s career. He has not pitched more than five innings since April 30 and has a 6.55 ERA.

Strasburg left his May 5 start with soreness in his back. A chiropractic adjustment temporarily alleviated the condition. He sprained his ankle in spring training and had compensated mechanically, and speculated that those changes may have caused the back trouble.

“You don’t know. It all starts from the ground, right?” Williams said. “So if he’s got a different feel from his ankle or he’s landing in a different spot or he’s compensating for it somehow going through spring training, then it’s gonna kind of come up the chain. The issue for us, and for Stras, is that he’s never had an issue with his arm — his elbow hurting or his shoulder hurting or anything like that. So it’s something to do with his back and his alignment, and it could be from the adjustment that he’s made where he’s landing and having to throw across his body more. We’ve looked at all that. We’ve exhausted everything.”

Strasburg’s pain earlier this month was on the right side of his back and lower. Friday’s was in his left trapezius muscle — the upper back — and crept into his neck.

“[The recent pain] is in a different spot. It’s his left trap. But something’s causing him discomfort in his upper back,” Williams said. “…”We gotta figure it out though,” Williams said. “Because it’s not allowing him to be free to pitch and do the things he wants to do. Certainly, when he goes out there and has to come out after the first, it puts our bullpen in a spot. But most of all, it’s frustrating because he’s not able to do what he wants to do.”

Williams said the fact that he has had pain on both sides of his back is a sign that the issue isn’t “anything structural,” as in bone damage, though the team does not know for certain. Strasburg will see team doctor Wiemi Douoguih on Monday.

“He’ll see all the great doctors, all the great trainers and therapists,” Williams said. “Hopefully get over the hump so he doesn’t have to have the issue again this year.”

Strasburg will continue to throw while he is on the disabled list, so he will not be completely shut down — at least until a treatable injury is diagnosed.

In the meantime, the Nationals will have to fill Strasburg’s slot in the rotation, which comes back around Wednesday against the Blue Jays at Nationals Park. Taylor Jordan, who relieved Strasburg for 4 2/3 innings Friday night, could be a candidate. Right-hander A.J. Cole started for Syracuse on Friday night, and would therefore also be on turn.

Hill, 26, made his major league debut last season. In three outings, one start, he pitched nine innings and allowed nine earned runs. He is 3-2 with a 4.35 ERA at Syracuse, but said Saturday he feels his stuff — which is anchored by a sinking fastball — was getting crisper and his approach better in recent starts.