In five games, few pitchers in baseball have been better than Nolan McLean the first two times through an opposing order.

Among 116 qualifiers entering play Saturday, when the Mets were rained out, his .143 batting average against ranked fifth, his .211 on-base percentage against sixth and .248 slugging percentage against ninth with a 2.67 ERA.

The third time through the order, however, one of the best pitchers in baseball morphs into one of the worst in the early going.

Among 100 pitchers who had logged at least four innings of work against an order a third time through, his .348 average against ranked 88th, .375 OBP against 78th and .609 slugging percentage against 91st with a 10.80 ERA.

The next step for McLean’s ascent has become obvious: He needs to maintain his stuff and keep opposing hitters guessing deeper in games.

“At the end of the day, I just got to execute pitches a little bit better,” McLean said Tuesday. “Maybe do a little more homework on how guys are adjusting to me.”

The 24-year-old, technically a rookie, was speaking after taking a perfect game into the sixth inning — for a second time this season — in a matchup with the Twins, who then began to figure him out.

In the sixth, Minnesota recorded its first hit, a Matt Wallner single, before turning the order over.

Leadoff hitter Byron Buxton, in his third time seeing McLean, smacked a two-run homer.

Further trouble awaited McLean an inning later, when a Kody Clemens double and RBI single from Luke Keaschall ensured that on a night McLean’s stuff was excellent, his stat line (6 ²/₃ innings, three runs) was ordinary in what became a loss.

Every pitcher becomes more hittable the more he sees an opponent, whether due to familiarity or fatigue.

McLean — whose breaking pitches often seem inhuman, such is his ability to spin a baseball — can begin making his adjustments when he pitches Game 1 of the doubleheader against the Rockies at Citi Field on Sunday.

“These [opposing] guys are getting paid to play baseball for a living, too,” McLean said last week. “… But I’ve got to execute better at the end of the day.”

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Sunday’s single-admission doubleheader will begin at 1:40.

Game 2 will start 30-45 minutes after the end of Game 1.

Tickets to Saturday’s game are not valid for admission to the makeup doubleheader.

Fans holding a ticket in their account for Saturday’s game will receive a digital voucher that will be accessible Sunday morning.