This morning I wrote that Oswaldo Arcia was getting closer to returning to the Twins. Technically that's still true, I guess, although it might be longer than anticipated since Minnesota has optioned Arcia to Triple-A.
The most logical explanation is that Arcia was approaching his time limit for a rehab stint, and the Twins weren't comfortable with his progress, and as a result were forced to make a roster move in order to keep him in the minor leagues. It's pure speculation on my part, of course, but it's what makes the most sense when it comes to Minnesota deciding to keep its starting right fielder from Opening Day - especially with the talent currently available at the Major League level - in Rochester.
Taking that reasoning to the next logical step: if the Twins are so uncomfortable with Arcia's progress and recovery that they're willing to option him to Triple-A in order to keep tabs on him, why not just keep him on the disabled list? Possibly because the team doesn't think it'll take another 15 days to have a complete evaluation; if they want to watch him for another week and then send him back to the Twins, that's too long for the rehab stint but not long enough for the disabled list, either.
It's easy to shake the head and get offended at this decision and throw the hands up in frustration, but the Twins aren't going to keep one of their best and youngest potential stars in the minor leagues "just because." Arcia's career line at Triple-A is .311/.409/.569. As soon as the Twins think he's ready to go, he'll be back in uniform.