Adam Rosales' role with the Texas Rangers had decreased in recent weeks with the addition of Mike Napoli and the solid play recently of Rougned Odor and Elvis Andrus.
A big plus for Rosales was that he could play all four infield positions, but Napoli's addition takes away the need for him to play first base. The Rangers have a younger, more athletic guy who can play third, short and second in Hanser Alberto, and now he's a Ranger again while Rosales has been designated for assignment.
He now has to go through waivers, and if he clears waivers he can accept an outright assignment to the minors where he'd likely be a September call-up to provide bench depth. Or, if a team claims him, he could be traded, but that's not highly likely.
Rosales has found a home in Texas since the roller coaster stretch in 2013 when he bounced between Texas and Oakland in a sort of waiver-claim musical chairs.
As Jamey Newberg points out, Alberto's promotion takes his name out of the running for the player to be named later in the Will Venable trade, as a player to be named later can't be in the big leagues between the time of the trade and the time of the announcement.