OAKLAND, Calif. — Justin Verlander is expected to take a significant step toward his Mets debut Saturday.

The team’s co-ace, who is rehabbing from a strained teres major muscle near his right armpit, is scheduled to throw a bullpen session in Port St. Lucie, according to manager Buck Showalter. It will be Verlander’s first mound session since he was placed on the injured list.

Verlander felt soreness in the muscle during his final spring training start, and when the discomfort continued during a subsequent side session, he was sent for imaging that revealed a low-grade strain.

If he doesn’t incur discomfort following his Saturday bullpen session, the plan is for him to repeat from a mound within a few days before throwing live batting practice and then potentially getting slotted in for a minor league rehab start.

That pushes Verlander’s timeline to potentially pitch for the Mets into their next homestand at the earliest.

Showalter had previously said Verlander wouldn’t be available during this 10-game West Coast trip that began Friday night.

The Mets plan to insert a sixth starter into the rotation during their upcoming series against the Dodgers, allowing Kodai Senga an extra day of rest. Senga’s turn in the rotation otherwise would have occurred Wednesday, putting him on a fifth day schedule for the first time.

Senga, who allowed four runs in 4²/₃ innings in the Mets’ 17-6 blowout win over the A’s on Friday night, is now expected to pitch Thursday to open the four-game series in San Francisco.

Showalter said one possibility would be using a reliever for the start against the Dodgers and then adding an additional reliever from Triple-A Syracuse.

Left-hander Joey Lucchesi is already on the Mets’ 40-man roster and would be among the options to be recalled in some capacity.

Lucchesi last pitched Thursday for Syracuse. Jose Butto, who is also on the 40-man roster, is scheduled to pitch Saturday for Syracuse, likely removing him from the equation.

Friday was the first of the 43 games the Mets will play against American League teams this season under MLB’s new scheduling format, which has roughly doubled the number of interleague matchups.

The Mets entered play with a .509 winning percentage in interleague games, which ranked second among NL franchises.