For the second straight day, Neil Walker was not in the lineup for the Mets.

The slumping second baseman said ahead of Monday’s series opener against the St. Louis Cardinals, which was postponed and rescheduled as part of a doubleheader Tuesday, that he was not surprised thanks to a conversation he and Terry Collins shared in Miami last weekend, and he’s confident he’s healthy and capable of regaining his form.

But Collins thinks Walker, who is hitting .174 since June 5, could use more than one day to “refresh” and “recharge.”

“Mental fatigue can lead to physical fatigue,” Collins said. “The swings can start to get a little long, which we see. Even though they think they’re not tired — they don’t feel tired — they are tired. When your mind starts to fade, you’re in trouble.”

Whether or not Walker’s mind has started to fade, his offensive prowess certainly has. The second baseman, in his first year with the Mets, was hitting .315 with nine home runs and a .963 OPS after the Mets’ 6-1 win over the San Francisco Giants on May 1. Even on June 4, he was hitting .285 with 13 homers and an .870 OPS.

But in 37 games since then, Walker’s average has plummeted to .239, his OPS to .717. He has three home runs and two doubles in that stretch.

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And within that span, he has had one extra-base hit — a home run — and four total hits in his last 45 at-bats (.089 average).

Just how anemic has Walker’s bat been? In 37 games since June 5, Walker’s wRC+ (weighted runs created plus) is 39, according to FanGraphs.com. That stat essentially measures a player’s offensive impact in relation to the league average, which is 100, and each point above or below 100 represents a percentage point. So in his last 37 games, Walker has created 61 percent fewer runs than the league average.

“Nothing mechanically is going on,” said Walker, whose season-long wRC+ is 95. Daniel Murphy leads National League second basemen with a wRC+ of 165. “I just need to be more consistent with my approach.”

Collins did not share how much longer Walker will remain out of the lineup. But the Mets are not short on options that could replace him — in the short term or long term.

Kelly Johnson, hitting .250 with a .685 OPS, started at second base in Sunday’s win over Miami and Monday against St. Louis. Wilmer Flores has played just seven games at second this season, but he has acquired a fair amount of experience there in his career.

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Collins said Jose Reyes could fill in at second base as needed, too. Reyes was hitting .242 with three home runs before Monday’s game against St. Louis.

“He hasn’t taken a lot of ground balls there , but I’m not opposed or worried to him going there,” Collins said. “He’s played the middle of the infield most of his career. I think going to the other side of second base is not a major factor.”