(Julio Cortez/Associated Press)

“He couldn’t do that,” Manager Davey Johnson said. “That really distresses me.”

Morse took himself out of his rehab appearance Monday night after seven innings. He could not throw the ball from left field to shortstop, and it worsened as the game went on. Nationals team doctors will reexamine Morse and further test the lat Morse first strained March 6. Morse will not play Thursday, as the Nationals had hoped. Johnson said the doctors are “concerned.” When Morse returns, after more than a month of diagnoses and setbacks, they still don’t know.

“I wish I knew,” Johnson said. “I didn’t think it was even that serious. I was kind of looking forward to penciling him into the lineup on Thursday. That’s not going to happen. That concerns me.”

When Morse first injured his lat this spring, he insisted he wasn’t worried about it. General Manager Mike Rizzo said, “it’ll be a day.” But it has turned into a month-long search for answers that has included a platelet-rich plasma injection and a sonogram to check for further tearing. The extended recovery has left the Nationals, having scored three runs less three of their first four games, without their cleanup hitter and best slugger.

“I thought the initial reports we got on him was that it was healed,” Johnson said.

The strain still does not affect Morse at the plate. Before he left the game last night, Morse had gone 5 for 12 with a homer and a double in his rehab.

On Saturday, Morse took a break from his rehab assignment and traveled to New York to receive treatment from a specialist. Johnson called the visit “a personal decison” not mandated by the team.

The Nationals have yet to determine Morse’s course from here on out, waiting for the results from his tests today in Washington. On March 28, Nationals doctors used a sonogram to confirm the small tear in Morse’s lat had healed. If something deeper has gone wrong, would surgery come next?

“That’s a tough question,” Johnson said. “I really don’t know.”

Without Morse, the Nationals have played Mark DeRosa and Xavier Nady, who started this season a combined 1 for 16, in left field. Adam LaRoche has hit cleanup, but they really have no means to replace Morse’s offensive production. Last year, he hit .303 with 31 home runs. This winter, the Nationals signed him to a two-year, $10 million contract extension.

Morse’s setback continued a bleak stretch for the Nationals’ health. The Nationals are awaiting results from closer Drew Storen’s visit with Dr. James Andrews, fearing he will need surgery to remove bone chips from his elbow. Two other players, starter Chien-Ming Wang and center fielder Rick Ankiel, began the year on the disabled list.

Top prospect Anthony Rendon suffered a badly sprained left ankle Saturday night, and Monday night outfield prospect Brian Goodwin left Hagerstown’s game with a leg injury.

The Nationals expect Ankiel to return from his quadriceps tightness Saturday, the first day he is eligible to come off the disabled list. But “I’m not betting the house on it the way things have been going,” Johnson said.