The engine that propelled the 2019 U.S. women's national team to a second consecutive World Cup title was a four-pronged, well-balanced midfield collectively entering its prime. It was Rose Lavelle, Julie Ertz, Lindsey Horan and Sam Mewis, all 27 years old or younger, all positioned to run the sport for the foreseeable future.

But four years later, entering another World Cup year, with the USWNT set to embark on another title defense, the team's longtime strength has become its biggest question mark.

Neither Ertz nor Mewis has played soccer since 2021, and head coach Vlatko Andonovski indicated Wednesday, more definitively than ever before, that he doesn't expect either to be available this summer.

Mewis has been battling a maddening knee injury that required a second surgery last month. She revealed earlier this week that there is no timetable for her return, and Andonovski confirmed that Mewis is "not gonna be able to play in the World Cup because of the injury."

Ertz, meanwhile, gave birth to her son Madden in August, and is without a club as National Women's Soccer League preseasons get underway. Andonovski said that "time is running out" for the 30-year-old Ertz. "She's someone that we're probably not gonna be able to count on in the World Cup," he said.

And so here we are, five-and-a-half months out from the tournament, with more worries than answers.

"These are two big names that may be or will be absent from the World Cup," Andonovski acknowledged. "And this is something that we have planned for, and that's why we've tried different names, different players in these positions." His problem is that, thus far, none of those solutions have proven viable.

He spent most of 2022 pairing Horan and Lavelle with Andi Sullivan, a serviceable defensive midfielder but certainly not an Ertz replacement. That midfield three lacked bite and was exposed in three straight autumn losses.

In January, Andonovski piloted Taylor Kornieck in the defensive midfield role, and in a scoreless first half against overmatched New Zealand, the USWNT's lack of rhythm and structure was alarming.

So, what now?

"We're going to continue trying in this camp until we solidify the players that we believe will give us the best chance to be successful," Andonovski said.

One potential solution that seemed worth piloting was Sam Coffey, who, as a 23-year-old NWSL rookie, became a key cog for the championship-winning Portland Thorns. In four USWNT appearances last year, she flashed upside. She seemed to be the most well-rounded, pure "No. 6" that the U.S. had.

Then she didn't play in January. On Wednesday, she was left off the roster for February's SheBelieves Cup entirely.

"There was something else that we wanted to see in this camp, in these games," Andonovski said when asked about Coffey's surprising omission.

There is, fortunately, still time to sort through that "something else." There are three games this month — against Canada (Feb. 16), Japan (Feb. 19) and Brazil (Feb. 22) — then more in April and beyond.

But there is no longer hope that ready-made reinforcements will appear when required. For months, perhaps for over a year, any USWNT struggles were tempered by the assumption that Ertz and Mewis would eventually return, but that assumption gradually gave way to the realization that they won't.

Both had been mainstays in the U.S. midfield for half a decade. Ertz was a dependable destroyer, a defensive midfielder without parallel in the USWNT player pool. And Mewis had ascended to the top of it. She was U.S. Soccer's female player of the year in 2020. She had developed into one of the world's very best players, galloping box-to-box at will.

They have left massive shoes to fill, proverbial shoes that, in all likelihood, no single player is capable of stepping into.

The solution will have to be in part schematic. Their absences will change the way the USWNT plays and what it is capable of. Those changes certainly don't disqualify the USWNT as World Cup contenders. There is still time to evolve, still time to emerge as a reformed force. And there are still other injured players who should return — Catarina Macario, the team's brightest star, is expected back in April.

But they have become Andonovski's biggest challenge as mid-July and World Cup kickoff near.