I'd normally agree with this assessment and your pessimism is fair, but Yo hadn't played for two years prior to his minor league debut. He also entered the season with a shoulder injury that limited him to DH duties early in the season. Add that into him having to acclimate himself to another country- he had a lot to deal with.
You're right on his big league tools...and I think he's going to show that immediately this spring. If he explodes in AA to start the year I don't think he'll be in the minors very long. I'd actually put Yo as the WSox top prospect as I feel he'll be making his case to be their RF by late May if they don't splash via trade or in FA.