Gentry hurt himself sliding into second base on July 24, and hasn't played since then. Anthony Andro suggests that Gentry could go on the 7 day concussion d.l., rather than the regular d.l., which would mean he could come back on August 1. Otherwise, Gentry would not be eligible to return until August 9.
So, why Teagarden instead of Leonys Martin? My guess is that the Rangers don't feel Martin is ready to contribute in the major leagues, and don't see the point of having him sit on the bench and watch and not be used. With Teagarden up, we'll probably see Endy Chavez playing centerfield the majority of the time, with some occasional Mike Napoli at first base/Mitch Moreland in RF alignments, and more Napoli coming off the bench as a pinch hitter.