With Matt Wieters starting the second half of the season on the DL, Craig Tatum will move up to assume the starting spot. Catchers can't play ever day though, and with Josh Bell being the guy called up to take Wieters' roster spot, that leaves Jake Fox to take the day games after night games. Fox has some experience behind the dish, with 12 games at the position in the majors and 276 in the minors (the most at any position). Will he be able to handle himself OK as back-stop?
In his brief time there for Oakland this year, Matt Klaassen has Fox as 0.7 runs above average defensively - with positive run values on throwing errors (he made one), passed balls and wild pitches allowed (none of the former, two of the latter), and caught stealings (nabbed two of three). Obviously 9 games isn't near enough to judge his skills though. In the minors, he caught 39% of potential basestealers and allowed 68 passed balls, and his Total Zone Catching Runs relative to average is -15, or about -7.6 per 140 games. So he's probably not great back there, but not amongst the worst catchers in baseball (those guys come in at 10 or more runs below average). And obviously over two or three games the difference between Fox and Tatum or Wieters are very likely to be negligible.
Jake's ability to fill in behind the plate in a pinch was one of the pluses to picking him up in that trade - it's handy (and his bat plays much better there).
Edit: As The Oriole Way noted in the comments, it probably makes some sense to just have Fox as the starting catcher and leave Tatum as the back-up. The difference in offense between the two (with the edge going to Fox) is greater than the edge in defense very likley. Over a couple weeks - with both guys starting some anyway - it's probably not going to make much of a difference though.