The Nationals will place cleanup hitter Michael Morse on “total shutdown mode” for six weeks, General Manager Mike Rizzo said, leaving an offense in need of punch without its best slugger for perhaps the entire first half of the season.

Morse re-aggravated the right lat muscle he first strained March 6 on Monday night during a rehab appearance with Class A Hagerstown. The Nationals had expected Morse to make his season debut Thursday in their home opener. Instead, they are left to wait on a hitter who crunched 31 home runs and hit .303 last year before a two-year, $10 million contract extension.

“We tried to ramp him up, get him ready as quickly as we could, because he’s a big piece of our lineup,” Rizzo said. “He just re-aggravated it. So now we’re going to have to be ultra cautious to make sure that he’s right for the majority of the season.”

The Nationals have “no viable surgical option” for Morse, Rizzo said, and so Morse will only rest. The Nationals have not placed a timetable on how quickly Morse could prepare for the season following his six weeks on the sidelines. It seems a conservative estimate that Morse will miss the season’s first 50 games.

“It’s frustrating,” third baseman Ryan Zimmerman said. “Mike is disappointed. With the year he had last year and how hard he worked in the offseason, how much he wanted to come back and continue to just keep that roll he had from last year, it’s tough. The hardest thing for him is to be patient and make sure he’s completely healthy.”

Without Morse, the Nationals have played Mark DeRosa and Xavier Nady in left field. They will soon add center fielder Rick Ankiel, who has spent the season’s first week on the disabled list with tightness in his left quadriceps, to their outfield mix. Ankiel will play for Class AAA Syracuse tonight and, if all goes well, will work out with the Nationals on Friday and return Saturday.

Rizzo said Morse’s injury will not affect top prospect Bryce Harper’s timetable to arrive in the major leagues, but Rizzo acknowledged Harper could eventually help fill the hole left by Morse.

“Anytime you lose a 30-homer, 90-RBI guy or more, it’s tough to replace,” right fielder Jayson Werth said. “With that said, we’ve got to pick it up, continue to do the things we’ve got to do until he gets back. We’ll be alright.”

The Nationals feel content about their internal options to replace Morse, but they will also consider outside choices. The Nationals scouted Diamondbacks outfielder Gerardo Parra during spring training, but the Diamondbacks were unwilling to move him for a price the Nationals deemed reasonable, and that trade seems unlikely.

“We’re comfortable with the people we have,” Rizzo said. “If there’s a viable option, if there’s a guy that fits what we’re trying to do this year and long term, we’re always open for that. We like the options that we have here. We’ve got a guy in Syracuse that’s an option for us. We feel good about where we’re at. … We’ve got in-house candidates that we feel comfortable with. It doesn’t change where we’re at with Bryce’s timeline at all.”

When Morse hurt his lat in the first day of spring training, the Nationals expected he would miss a few days, a week at most. He appeared in two games, March 12 and 13, as a designated hitter. But he never felt strong throwing the ball, and Monday night he re-aggravated the strain while making a throw in cold weather.

Manager Davey Johnson said the left field situation would be “a work in progress,” with playing time decided by how players perform. But he said he believed the Nationals had enough power in their lineup without Morse.

“I think if you look at the organization and the lineup, everybody in the lineup is capable of going longball,” Johnson said. “I don’t think that’s the critical factor in taking 30 bombs out of the four hole.”