The beleaguered back end of the Mets’ rotation appears set to get a boost. Barring any setbacks, starter Shaun Marcum should make his season debut Saturday against the Phillies.

The veteran right-hander threw five scoreless innings in an extended spring training game in Port St. Lucie yesterday. He gave up three hits and struck out four, and did not walk a batter in his 52 pitches. He then threw more in the bullpen to reach the club’s stated goal of getting him up to 70 to 75 pitches.

Should Marcum be activated and pitch Saturday, manager Terry Collins said Marcum would be held to a 90-pitch limit.

After inking a one-year, $4 million contract this past offseason, Marcum has been on the disabled list due to nerve inflammation in his neck.

Expecting Marcum’s arrival, the Mets designating struggling starter Aaron Laffey for assignment on Sunday and recalled reliever Rob Carson from Triple-A Las Vegas.

* The Mets inked right-handed pitcher D.J. Mitchell to a minor-league deal yesterday. The 25-year-old former Yankees farmhand will report to extended spring training.

After Mitchell had made four relief appearances for the Yankees, posting a 3.86 ERA in 4 2/3 innings, they shipped him to the Mariners last July as part of the package for Ichiro Suzuki. Mitchell compiled a 2.96 ERA in eight starts with Triple-A Tacoma after the deal, and this year he made one start for Tacoma, allowing three runs in four innings on April 8.

* The Mets have no intention of moving Lucas Duda up from sixth into the cleanup spot in the batting order. Collins said he doesn’t want to disrupt his outfielder.