All too often, we are reminded how human arms were not designed to pitch baseballs. The latest reminder came on Sunday afternoon, with Braves right-hander Kris Medlen on the mound for a spring training start. Everything appeared normal until Medlen induced a ground ball from Mets catcher Travis d'Arnaud in the fourth inning, after which Medlen gingerly rubbed his right elbow as he returned to the mound. After the second pitch to the next batter, Medlen hopped off the mound and immediately moved toward the dugout, holding his pitching elbow in apparent pain.

The club is calling the injury a forearm strain for now. As encouraging as that may sound, "strain" is merely another medical term for a tear. Medlen underwent Tommy John surgery in August 2010 and missed the majority of the 2011 season in recovery. Until Medlen undergoes an MRI on the arm, the possibility that he will require a second Tommy John surgery cannot be dismissed.

For Medlen, 28, such a diagnosis would be devastating. Although the list of successful repeat Tommy John survivors is growing, he would still face an uphill battle to even return to the major leagues, much less replicate his success of the last two seasons. At least he managed to stay healthy long enough to pick up the service time necessary to get paid, and Medlen will earn $5.8 million this season regardless of if he's able to suit up. As large as that sum is, it is little compared to the value Medlen has provided to the club since his return in late 2011.

To be specific, Medlen gave Atlanta 335 spectacular innings over the past two years, covering 43 starts and 39 relief appearance with a 2.47 ERA and 277 strikeouts against just 70 walks. Of pitchers to throw 250 innings or more over the past two years, only Clayton Kershaw has a superior ERA (at 2.17). Medlen was at his best in 2012, when he posted a 1.57 ERA over 138 innings, but he was still excellent over his first full season as a starter in 2013 with a 3.11 ERA (124 ERA+) and a sharp 3.3 K/BB.

Atlanta manager Fredi Gonzalez sounded relatively optimistic following the game, as he told reporters, "I think after everything kind of settled down, we're optimistic that it's nothing really major." The club better hope Gonzalez is right, because this team lacks the kind of pitching depth that has come to define Atlanta baseball over the past couple of decades.

Without Medlen, the Braves rotation appears to be as follows: Mike Minor, Julio Teheran, Brandon Beachy, Alex Wood (in some order) and a gigantic question mark. David Hale is currently listed as the Braves' sixth starter (including Medlen), but he has 11 major league innings to his name and just 125 above the Double-A level. He had limited success in the minors until posting a 3.22 ERA for Triple-A Gwinnett last season and has never appeared on a top 100 prospects list, although if Hale were a success it certainly wouldn't be the first time the Braves have spun prospect straw into gold.

The bigger problem is that the Braves rotation was already on a shaky foundation. Beachy is a Tommy John survivor himself, had another minor surgery on his right elbow last September, and has started just 18 games since breaking out in 2011. The club has hesitated to commit to Wood in the rotation, due to questionable mechanics and iffy secondary stuff. Wood has started just 35 games across all levels of professional baseball, and was significantly better in relief (2.08 ERA, 4.6 K/BB) than as a starter (3.54 ERA, 2.5 K/BB) in 2013. The Braves would certainly be happy if he replicated his 2013 rotation numbers in 2014, but there are significant questions as to his ability to do so and stay healthy over 200 innings.

As usual, the Atlanta organization is rich in minor league pitching prospects, but the majority of the club's top talent appears far from contributing at the major league level. Lucas Sims, the club's top prospect, is just 19 and has yet to pitch above A-ball. Mauricio Cabrera is less than a year older and has also yet to reach Double-A. J.R. Graham is 24 but struggled with injuries last year and hasn't pitched at Triple-A. Due to his age, however, he is likely the most realistic reinforcement of the club's top prospects.

The Braves have been happy to hold onto excess pitching in the past because they know arms can collapse without warning. They were able to withstand the losses of pitchers like Tim Hudson and Tommy Hanson in recent years because the likes of Medlen and Beachy and Wood were able to step in as capable replacements. But even if Medlen is able to avoid the disabled list, this appears to be the shallowest Braves staff in recent memory.

It should be no surprise that within a couple hours of Medlen's injury, news broke that free agent starter Ervin Santana would be stretching out his free agent decision for "a few more days." Earlier reports indicated the Blue Jays were expected to sign Santana to a one-year contract by the end of Sunday if he had not received a stronger offer. Santana and his agents may hope the extra time will induce the Braves to jump into the bidding. It would be an uncharacteristic move for Atlanta -- it would cost them their first-round draft pick -- but Santana is clearly the best talent remaining on the pitching market.

Santana or no, Medlen's scary moment Sunday puts Atlanta's suddenly worrying lack of rotation depth under the spotlight. No club has been better in recent years of making something out of nothing at the bottom of a rotation, but 2014 is shaping up to be one of their greatest challenges yet.