The 2015 MLB season is nearing the month of June. The St. Louis Cardinals rank atop the league in ERA (2.79), but their starters' statistics have witnessed a decline since Adam Wainwright tore his Achilles tendon in late April.

In April the Cardinals' rotation was 11-4 with a 2.85 ERA and 1.172 WHIP through 21 starts. In the month of May the cast of starters has still been solid, but they've clearly suffered without Wainwright. They're 7-7 with a 3.22 ERA and 1.335 WHIP in 24 starts. (All statistics provided by Baseball-Reference.com.) It's not too drastic of a decline, but it's noticeable enough.

Jaime Garcia logged a solid outing in his first start since June 20 of 2014 over the weekend, but the Cardinals failed to score runs. Garcia surrendered two earned runs on five hits and five walks over seven innings in a 5-0 loss to the New York Mets. However, Garcia's longevity is still a big question mark - he has made only 37 starts since 2012 and 17 since 2013.

As for Carlos Martinez, he's been impressive as of late, but manager Mike Matheny isn't sure what to expect out of the young right-hander on a given night. In his last two starts he's 1-0 with a 0.00 ERA and 13 strikeouts in 13 1/3 innings, but he was 0-2 with a 10.29 ERA and 19 strikeouts in his three starts prior (14 innings total).

Michael Wacha, John Lackey and Lance Lynn have pitched well for most of the season, but the continuity of the rotation is still something to be concerned about. Wacha is still coming off a mysterious shoulder injury that sidelined him for nearly three months last season too.

General manager John Mozeliak has tried to mix-and-match some arms from Triple-A Memphis, but that hasn't gone too well either.

"In their four starts, lefties [Tim] Cooney and [Tyler] Lyons allowed 25 hits and walked six in 15 1/3 innings," Joe Strauss of the St. Louis Post-Dispatch wrote last week. "They surrendered 13 runs, 11 earned, for a 6.46 ERA. The greater indictment, however, was the duo's inability to push through five innings, a tendency that vaporized the bullpen."

"At some point the Cardinals' brain trust could devise a way to reapportion innings among its younger hurlers. [Marco] Gonzales' return from a shoulder issue is a factor," he added on Monday.

In fact, Gonzales is undergoing "a series of tests on Tuesday to determine the cause of recurring soreness in and around his left shoulder," according to Derrick Goold of the St. Louis Post-Dispatch.

Is a trade for a low-cost veteran pitcher or even a top-tier starter in the cards with the uncertainty surrounding the longevity and consistency of the rotation?

"The guess here is Mozeliak will pull off a trade for a starting pitcher before July 31. Names like Kyle Lohse, Aaron Harang and Scott Kazmir are popular now and may attract company as more teams recede from contention," Strauss added.

"Adding an arm while benefiting from a healthy Garcia or Gonzales might allow current No. 4 starter Carlos Martinez to return to a set-up role."

Martinez has shifted to a starter this season after coming out of the bullpen in 2014. It's unknown what Matheny's plan is for the youngster, but one can only assume of the inconsistencies continue that Martinez could become a candidate for the bullpen.

Lohse, Harang and Kazmir would perhaps be among the cheaper options on the trade market. There are a number of expensive veteran aces, including Cole Hamels, who the Cardinals have been linked to for quite some time now through trade rumors.

"St. Louis, considered the best organization by one competing GM (along with the Giants), is seen as a team that will need a starting pitcher since Carlos Martinez isn't thought to have logged enough innings to go deep into the season," writes Jon Heyman of CBS Sports. "However, to be involved for someone like Hamels, rivals think St. Louis might have to trade off its major-league roster."

Mozeliak and his front office team are protective of their farm system because they build the franchise by developing and cultivating young talent. It's more than likely they'll have to give up prospects for Hamels, and Mozeliak may not want to do that.

However, Mozeliak will need to ensure the starting rotation can last for the entire season by remaining healthy and dependable for a majority of their outings. He has gotten a taste of what could potentially happen and he likely wants to avoid that, which could mean a blockbuster trade will be considered in the next couple of months if things continue to get worse.