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Earlier today, the Cardinals announced that they activated shortstop Ruben Tejada, and sent Greg Garcia down to AAA. With Jhonny Peralta on the DL until at least June, St. Louis needed a replacement shortstop, and wound up signing Tejada after the Mets released him.
At the time, it appeared that he would be the starting shortstop for the foreseeable future, while Jedd Gyorko would serve as Tejada's backup. However that all changed when Tejada injured his quad during spring training. In his place, Aledmys Diaz has been playing everyday, and has been fantastic over the course of his first 10 games at the major league level.
In (small sample size alert) 34 plate appearances, Diaz is hitting .406/.441/.813 with a wOBA of .521 and a wRC+ of 228. He's almost been worth one win already (0.7 fWAR) and by offensive and defensive metrics, Diaz has been more than capable of handling shortstop.
While he's not going to hit above .400, or finish the 2016 season with a slugging percentage higher than Bryce Harper's 2015 campaign, Diaz has undoubtedly shown the Cardinals that he deserves to hold onto the starting job for the time being.
When the Cardinals signed Tejada, he was supposed to be nothing more than a stopgap. In 2185 career PA's, Tejada's slash line is a .255/.330/.323, and by wRC+, he's never been an above average major league hitter. That's not a knock on Tejada as a player, but rather an indication that there isn't much upside to letting him play over Diaz.
Peralta is a free agent at the end of the at the end of the 2017 season (as is Diaz), and if the Cardinals have a potential long-term replacement, they need to find that out sooner rather than later; and there might not be a more perfect time to figure that out than right now.
At just 25 years of age, Diaz could be coming into his own, and his best years should still be ahead of him. 34 PA's is hardly enough to forecast anybody, but considering that Diaz posted a wRC+ of at least 108 at every single minor league stop, it would not be surprising to see him blossom into an MLB regular.