Martin, as folks here well know, is a centerfielder who defected from Cuba last year, and who was signed to a 5 year, $15.5 million deal by the Rangers earlier in 2011.
Upon signing, Martin was viewed as a player with good speed who would be a plus defensive centerfielder, but who had questions about his hit tool. While Martin tore it up in AA, he's struggled since his promotion to AAA Round Rock, posting a .263/.316/.314 line in Round Rock.
There was some speculation earlier this year that Martin would be the Rangers' starting centerfielder in September and the playoffs, but that has cooled down of late. However, with Nelson Cruz being removed from last night's game with a hamstring injury, the Rangers apparently feel that they need another outfielder up with the major league club, and Martin is the guy on the 40 man roster who would be the best fit.
At this point, however, I don't see him getting significant playing time.