As has been discussed, the Rangers were going to have to remove someone from the 25 man roster to make room for Justin Grimm on the active roster for his start today. The original plan was to put Koji Uehara on the disabled list, but Uehara's spot was taken by Leonys Martin, who was called up yesterday to fill in while Josh Hamilton recuperates from an intestinal virus.
So instead, according to folks breaking the news on Twitter, the Rangers have optioned Yoshinori Tateyama to AAA Round Rock to make room for Grimm. Tateyama is a sidearmer with big platoon splits, but instead of using him as a specialist, the Rangers had been using him as a sort of long man/garbage time pitcher, which doesn't make the best use of his skills. With Tateyama going down, the Rangers' bullpen is now at 6 pitchers, and likely will continue to be at that number until at least Tuesday, when Hamilton is supposed to be ready to play again, and Martin will likely be sent down.
To make room for Grimm on the 40 man roster, the Rangers have transferred Neftali Feliz from the 15 day disabled list to the 60 day disabled list. Jon Daniels previously said this is strictly a procedural move, and doesn't reflect a change in the prognosis for Feliz, who is expected to start throwing next week.
You can read the write-up I did of Grimm this offseason here.