I'll tell you exactly why he won't pick Rio for the squad; because Rio won't listen to him and his "hoof the ****ing ball up the pitch" tactics. Rio is a ball playing central defender, the same as Daniel Agger at Liverpool. Uncle Woy doesn't like his CBs playing the ball out from the back. He told Daniel Agger, arguably the most technical center back in the PL, and I quote, to "get the ****ing ball up the pitch" when he played it from the back.
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That might have been the reason he initially chose JT over Rio, but ultimately it came down to the issue between the two. Rio said he wouldn't feel comfortable being with JT at the tournament because of the racism allegations, and Hodgson had a choice to make...one or the other.
He can't pick Rio after what happened to Cahill, simply because he didn't pick Rio originally due to the fact Rio and JT won't work together. If Rio had simply said, "yes, i can work with JT" he'd be on the plane to Poland/Ukraine (even though he is injury prone and older/slower). But he didn't. I understand Rio being upset, but he gave Roy Hodgson the answer to Roy's question by saying he wouldn't play with JT.
Granted, the entire England team selection is a headache. Start Gerrard/Lampard who have never worked well together in central midfield, who to pick out of the maybe three qualified center forwards to start two tournament games for England with Rooney suspended, Rio/JT fiasco and how to keep the team happy...it's a headache.
Someone mentioned earlier that best case scenario England lose on PKs in QFinal round? I say the best case is getting back on the plane the morning of June 20 (day after final group game). They won't get out of the group. Sweden and France will.