Then, in July 2012, less than two years later, the Cardinals traded Cox to the Marlins for relief pitcher Edward Mujica.
At 24, Cox remains a bit of a mystery. A challenge. A prospect who has not lived up to his expectations. Can he unlock the hitting ability that the Cardinals and scouts saw in college?
Cox is No. 13 on the Marlins' top 20 prospects list.
Because he signed late, Cox didn't get many plate appearances during his first pro season in the Gulf Coast League. He played in only four games and went to the plate 17 times.
The following season, Cox played at Class A Advanced Palm Beach and Double-A Springfield. He hit a combined .306 with 13 homers and 68 RBIs. He had 516 plate appearances, striking out 98 times.
Cox went to Triple-A Memphis to start the 2012 season. He hit .254 in 316 plate appearances. He had nine homers and 30 RBIs. Then, he was traded.
Following the trade, the Marlins sent him to Double-A Jacksonville.
Playing only a partial season due to injuries, Cox hit .253 for Jacksonville. In 106 trips to the plate, Cox hit only one home run.
At the end of Spring Training and prior to this season, the Marlins waived Cox from their 40-man roster. He went unclaimed.
Returning to the Marlins, Cox was again sent to Double-A. He hit .264 with three home runs and 29 RBIs.
Somewhere along the way from being a hitting machine in college to his current status in his second organization, Cox changed his approach to hitting.
When I saw him in the 2010 Arizona Fall League following his selection by the Cardinals, Cox was a "swing-from-the-heels" hitter. He was an aggressive pull hitter with home runs on his mind.
It appears Cox is now concentrating on making contact with a shorter swing and quicker hands through the ball. I think it was a good decision on his part to be less aggressive and more measured.
At 5-foot-11, 225 pounds, Cox is a stocky guy with a thick upper body. A left-handed hitter, he has the ability to drive the ball to his pull side with some authority. However, he doesn't yet have the power to knock the ball over the fence with any regularity. Given his age, Cox's physical development is likely complete.
Using mostly his arms and wrists and not his trunk in a rather flat swing, Cox limits the loft he can get on the ball. That's fine, provided he can find the holes and get his share of hits.
Defensively, Cox is adequate at third base. He makes the routine plays, but he lacks the agility and first-step quickness to make the extraordinary play.
The 2014 season could be a turning point for Cox. He has to become less puzzling and more productive.
The organization lacks depth at third base, and Cox may be a bridge player until this past June's first-round Draft pick, Colin Moran is ready to assume a role as the Marlins' regular third baseman.