This is the test for the rest of this Mets season: Get an idea for tomorrow with what’s happening today.

So, whether it’s Matt Harvey getting back on the Citi Field mound to throw a simulated inning before Tuesday night’s 5-4 win over the Rangers in hopes of a September return, or another start behind the plate for Travis d’Arnaud, it’s all about figuring out what’s best for next year and avoiding another nightmare season.

Not just the physical aspect, the mental aspect as well.

In d’Arnaud’s case it’s about focus and fight, and Tuesday night he took another step in the right direction, lining a two-run home run in the second inning, his first home run since June 27 and his first Citi Field home run in nearly a year, dating back to last Aug. 12.

D’Arnaud remains one of the biggest question marks for the Mets and he has performed better in the second half than the first, hoping to stay in the team’s plans.

It appears the Mets are leaning toward keeping d’Arnaud as their starting catcher because of the improvement he has shown in his batting average and his more focused approach to the game. D’Arnaud batted .223 in the first half, but has jumped to .295 the second half.

There is a long way to go, but there has been progress.

“He’s really stayed with his mechanics,’’ hitting coach Kevin Long told The Post before offering these heart-of-the-matter comments about where d’Arnaud needs to improve. “I feel like he has done a really good job of staying more consistent. We stay on him every day about doing the exact same thing.

“Reality has set in and he is saying, ‘I better get this done.’ You say, ‘How much longer can I be mediocre?’ He’s got a high ceiling, it’s time that he gets there,’’ Long said. “He’s got to push himself to be a better player, a better hitter.

Sometimes it takes players a little bit longer, hopefully in this case, it’s starting to sink in: ‘Man, I better get this done.’ ’’

Long knows it’s time for d’Arnaud to show improvement and consistency. The catcher will be 29 next season.

“Your window of opportunity can only last so long,’’ Long added. “In this game, somebody else is always coming along to try and take your job. You need to realize that. He’s too good to just throw at-bats away. Every pitch has got to matter. Every at-bat has got to matter. … When you get to that point, you become a much better hitter. He’s focused a lot more and he’s starting to understand pitch to pitch that he needs to fight.’’

Strong words and solid advice that d’Arnaud has taken to heart.

“I’m trying not to over-swing or overdo anything,’’ d’Arnaud said. “Just keep things simple.’’

He then offered this look-in-the-mirror assessment.

“There was a point where I think I was overthinking so much, trying to do more than I had to do instead of just playing the game. This has been an eye-opener.’’

His career depends on making those adjustments. Five years in, d’Arnaud owns a .245/.308/.398 slash line. At the time, d’Arnaud was the big get in the R.A. Dickey trade, but Noah Syndergaard has become the star of that deal.

D’Arnaud is quick to credit Long and assistant hitting coach Pat Roessler for his recent progress.

“I’ve got two good hitting coaches, honestly. They stay on top of me. If I show up in the cage one day and try something new, they walk up to me and say, ‘Hey what are you doing? You’re better than this.’ Then it’s like, ‘OK, you’re right.’ ’’

D’Arnaud is hitting .289 in Mets wins, .195 in losses. His bat is key.

“For me, it’s more about winning,’’ he said. “The games I catch I try to win. As far as [what] the future holds, I have no control over that. But if I focus in on this moment and every day like it’s the most important pitch or the most important anything, then good results will happen.

“It’s mentally exhausting,’’ d’Arnaud admitted of his new approach. “I fall asleep so much quicker now than I used to. My mental goal is to grind it out.’’

Focus and fight every night. Don’t give away an at-bat.