I think he looks at Kane and Alli and thinks Poch will do the same for him plus if he does manage to break into the Spurs starting 11 (the position on the right is achievable) then he has a far better chance of not just making the England squad but the team.He won't get as big a contract at Spurs. In fact, probably no where near it. This has surely always been about him not getting into the England side.
We need to keep this lad - get rid of the likes of McCarthy for £20m to fund the Gylfi purchase if needs be.
I hate to say it but while it's a gamble it's a good move for his career and at 24 this world cup will potentially be the only one he will be at his peak. As things stand if he stays with us he probably wouldn't make the squad. By not picking or playing him even when he's performed well Southgate has effectively forced his hand.
Which is why signing a load of England youth internationals may not be the wisest of moves. Unless we are in the top 6 they will all want to move for this same reason.
I wonder what he would do if we just decided to keep him negating that reason.