As if Mets fans needed another reminder of just how much of a train wreck this season has turned into, here comes the biggest one of all: The Dodgers will be at Citi Field when the Mets open their homestand Friday.

It comes on the heels of another lost series, this one to the Brewers, where David Stearns’ old team is battling the Dodgers for the best record in the sport.

And now the same Dodgers the Mets were supposed to emulate when Stearns arrived in Queens and paired with Steve Cohen’s money are in town to show off just how big a gap there is between the two franchises.

Because instead of a battle of coastal powerhouses, the series that opens Friday is another example of just how far the Mets are from reaching their goal.

The Dodgers have the best record in the majors (65-38) and stand to get even better over the next two weeks.

Even before they figure to add improvements by the Aug. 3 trade deadline, they should get their closer, Edwin Díaz, back.

The ex-Met is expected to return to the bullpen within the next few days — although not likely by this weekend — after surgery to remove loose bodies in his elbow.

Kiké Hernández has been out with a strained oblique and could be back this month.

Meanwhile, the Dodgers have not just overcome the absence of those two key players, but catcher Will Smith is out indefinitely with a neck injury and Shohei Ohtani has been dealing with left knee inflammation that’s kept him off the mound.

Blake Snell and Tyler Glasnow are expected back in the rotation next month.

The Dodgers recently shrugged off a sweep at the hands of the Diamondbacks in Los Angeles and won series against the Yankees and Phillies — both on the road.

As for the Mets, they haven’t won as many as three straight games since the end of May.

They are a season-high 17 games under .500, 17 games out of first place in the NL East and 10 ½ out of the final NL wild-card spot.

And now they’ve forced themselves into sitting one of their only bright spots — either Carson Benge or A.J. Ewing — on a regular basis to get Luis Robert Jr. in the lineup to potentially increase his trade value.

“I understand why they’re doing it,’’ one National League scout said of the decision to try to play Robert regularly now that he’s finally returned from a back issue, “as long as it’s just until the [trade] deadline. But it still doesn’t look great. Having those two rookies out there is about all that’s worth paying attention to, but their development isn’t gonna be impacted by a few days out of the lineup in July. It just can’t go on later [in the season].”

It’s still another example, though, of how this season has gone off the rails for the Mets, as the scout noted the number of questions they have up and down their roster, which will continue at least for the foreseeable future.

“Who’s their first baseman next season?” the scout said. “Second baseman in two or three years? Third? They’ve shown you can’t just trade or sign those guys that easily. The Dodgers have shown the ability to find the right guys to fill out their team. The Mets haven’t.”

And that will be on display in full effect for the next three days.

While Nolan McLean is starting Saturday, he’s sandwiched between Sean Manaea and Freddy Peralta.

By the end of the 10-game homestand that begins Friday, the Mets will look considerably different and certainly not any better.