T.R. Sullivan reports that Kason Gabbard will start the season in the majors as the Rangers #4 starter, and Luis Mendoza will begin the year in Oklahoma, getting called up on April 12, when the Rangers need a #5 starter for the first time.

I don't really have much to add beyond that.  There was some thought that Mendoza might get the #4 spot, instead of Gabbard, but it seems like if you thought Gabbard was good enough to have the #5 spot nailed down to begin with, he's good enough to start the season in the majors instead of Mendoza.  If Gabbard were having mechanical issues or other problems such that he could have used some time in the minors to get his act together, it would have made sense to flip-flop them, but at this point, this appears to be the right move.

And of course, I'm sure everyone is glad that the #5 starter is not Sidney Ponson...