PHILADELPHIA — The Red Sox are back to square one when it comes to injured starter Tanner Houck.

Houck felt arm soreness after his last rehab outing for the WooSox on July 9 and has been shut down from throwing, manager Alex Cora said Monday. There’s no timetable on his return.

Houck, who first went on the injured list due to a flexor pronator strain in his throwing arm on May 13, was pulled off his rehab assignment Friday and remains on the IL. After pitching five times in the minor leagues over a three-week period, he appeared close to returning before re-aggravating the injury.

The Red Sox hope rest will help Houck bounce back. Tests revealed no significant damage.

“He’s going to stay on the IL,” said Cora. “They checked on him. There’s no surgery needed. Just rest. Re-set him now, get treatment and see where we’re headed after that.”

Houck, an All-Star last season, looked lost in the first six weeks of the season. He posted an 8.04 ERA and allowed 57 hits in 43 ⅔ innings, blowing up twice in a four-week span (April 14 in Tampa and May 12 in Detroit). He had mixed results in his rehab outings but had better numbers toward the end, pitching five shutout innings for Double-A Portland on July 4, then striking out six and allowing a single run for the WooSox in a 4 ⅓ inning outing in Rochester five days later.

Houck’s 30-day rehab window expired over the All-Star break. It was clear to the Red Sox then that they could not activate him. For now, they’re hoping treatment on his arm will help put him in a place where he can resume throwing. Cora didn’t have a timetable for Houck’s return.

“I can’t give you an answer right now,” Cora said. “This happened during the week. He felt it in his last one. He saw the doctors and all that stuff. We’ve got to go to Step 1, I guess.”

Rookie Richard Fitts will stay in the rotation with Houck and Hunter Dobbins (torn ACL) out.

“He’s part of the rotation,” Cora said. “He has done an outstanding job since last year, giving us a chance to win games. We expect the same thing out of him.”

Bullpen arms progressing

Cora is encouraged by the progress of righties Justin Slaten (right shoulder inflammation) and Liam Hendriks (right hip inflammation), who are both playing catch and getting stretched out. Right-handers Luis Guerrero (right elbow strain) and Zack Kelly (right oblique strain) have progressed to facing hitters for the first time since hitting the IL.

“Slaten is playing catch and he’s feeling better. Hendriks is doing the same thing. Guerrero threw a live (BP). Kelly threw a live today and will throw another one Thursday,” Cora said. “Everyone’s progressing. They’re feeling better. I don’t know when they’re going to go on a rehab assignment if needed, but we’re in a good spot.”

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