While the Rangers announced before tonight's game that Yu Darvish was going on the disabled list, they did not announce who would take his place on the active roster.
However, per the beat writers on Twitter, Joseph Ortiz is being recalled from Round Rock to replace Darvish. He will join the Rangers' bullpen for tomorrow night's game.
There is speculation that Josh Lindblom will be optioned tomorrow, with another pitching coming up to supplement the bullpen. However, there are not a lot of choices, at this point...the only healthy pitchers on the 40 man roster who are still in the minors are Wilmer Font, Neil Ramirez, and Justin Miller. The Rangers could also purchase the contract of someone like Ben Rowen or Jake Brigham.
With Darvish going on the d.l., the Rangers need a spot starter for tomorrow's game against the Orioles, and Ross Wolf is getting the nod. He will face off tomorrow against Miguel Gonzalez.