In the best-laid plans, a veteran cast of MLB starters were going to serve as protection for Kodai Senga early this season. Buy him time to familiarize himself with a different baseball and a better crop of hitters and often pitching on the fifth day rather than once a week like in Japan.

But in a game of Rotation Survivor, Senga became the last starter standing in April. The Mets prematurely need him to be both good and a workhorse. And he is not yet ready for that dual responsibility.

Senga did not pitch the Mets out of Wednesday’s 4-1 setback against the Nationals. By designation, he “lost” the game. But that was more about a Mets defense that came in with the second-fewest errors (seven) in the majors and committed three more. And an offense that went 1-for-13 with runners on base and struck out 15 times after whiffing 13 times against the last-place Nationals 24 hours earlier. The Mets have one run in the two contests.

Those were the major culprits as the Mets lost a fourth straight game for the first time in Buck Showalter’s year-plus as manager. Senga battled enough to hold the Nationals to two runs in five innings, inducing two key double plays out of Washington catcher Keibert Ruiz and producing six of his seven strikeouts with men on base — five of the punchouts with runners in scoring position.

But it was only five innings as he walked four, worked long counts and endured a lot of wearying on-base traffic. And the Mets hunger for more from a currently broken rotation. Showalter said he is determined not to overtax his pen. Pitching coach Jeremy Hefner, though, admitted concern about doing just that without getting length from his starters.

The lack of length is understandable. Jose Quintana and Justin Verlander have yet to pitch this year. Max Scherzer (suspension) and Carlos Carrasco (elbow) are currently unavailable. The hope was that those four would ease the adjustment period for Senga with either Tylor Megill or David Peterson offering a sixth starter occasionally to ease the load on the whole group, notably Senga.

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Instead, the veteran quartet has started just seven of the Mets’ first 25 games. They have had just one starter get an out in the seventh inning, Joey Lucceshi, who on Thursday will try to help the Mets avoid a sweep against the lowly Nationals.

The Mets want to believe that the cavalry is coming. Scherzer threw a four-inning simulated game at Citi Field on Wednesday and is due back from his suspension to start Monday against the Braves. Verlander (shoulder) has a minor league rehab start slated for Friday, which tentatively has him in line to make his Mets debut Wednesday in Detroit.

But Scherzer’s early run-ins with the pitch clock and authority and Verlander’s with injury — and the reality that Scherzer turns 39 in July and Verlander is 40 — has at least turned the knob up somewhat on concerns whether they can be the workhorse aces of their career; essentially honor the reason the Mets made them co-owners of the largest ever per-annum contracts in MLB history.

“Their mentalities will allow them to overcome any short-term issues,” Hefner said. “They are so good upstairs [mentally] that any shortcomings on the field, they will adjust. They are not 25 or 28 anymore and able to throw 100 mph with every single fastball. Their velocities have come down. But their pitchability and how they overcome these little things, that’s still there. So I expect them to be the pitchers they have been.”

Scherzer faced Met minor leaguers from Single-A and Double-A in a simulated game and “looked like himself,” in Hefner’s assessment. When asked if Scherzer is working through injury, particularly with a problematic back, Hefner said, “He’s dealing with naggy things that most guys are dealing with and using the time during the suspension as a time to get over those types of things.”

Of course, “naggy things” become more troubling with age. And when I asked Showalter if he expects Scherzer and Verlander to perform at a peak level, he initially said he didn’t want to answer it before adding, “I hope so.”

Showalter is dealing with an issue that once beset predecessors in his job — the dream of what it would look like if the Mets could have Jacob deGrom, Matt Harvey, Steven Matz, Noah Syndergaard and Zack Wheeler all healthy and performing well simultaneously. That quintet only ever managed two rotation turns as a unit.

With Quintana (stress fracture of his ribs) not a possibility before midseason, the Mets may never see the fivesome intact. And with Carrasco a real worry about returning at all, do they even get a foursome?

It means the Mets need Senga to adapt quickly to his new environment while Scherzer and Verlander return quickly to being the pitchers of old, rather than old pitchers.