These moves aren't unanticipated, and in last night's podcast recording, I predicted these were the two roster shuffles we would see. Mark Lowe, when healthy, has been one of the team's best relievers, and thus it isn't surprising that since he's now healthy, he's replacing Tateyama, who was the last man added to the bullpen for the ALCS.
The addition of Treanor is, I think, driven in no small part by the fact that the Rangers are potentially playing four games under N.L. rules, and thus will need more pinch hitting options for their pitchers. The best righthanded bench bat available would probably be Torrealba, but there's going to be a reluctance on the part of Ron Washington to use Torrealba as a pinch hitter without a third catcher available on the bench. Treanor gives Wash a little more bench depth and a better comfort level using Torrealba to hit for a pitcher in the N.L. parks, if need be.
Finally, to make room for Treanor, the Rangers are dropping Uehara. Uehara was a trade deadline acquisition from the Orioles, a guy who Texas gave up Chris Davis and Tommy Hunter to land, but he's struggled with the long ball since arriving in Texas, has spoken about his lack of comfort in a new environment, and after his most recent playoff meltdown, acknowledged that it wouldn't be surprising if the coaching staff didn't trust him at this point.
Uehara was expected to be a key part of the team's playoff bullpen, but his performance has been such that I don't think anyone is comfortable with him pitching in a big situation against the Cards.