Alike to any potential transfer dealing any Club ever makes - there is a risk.
And by the way, if you were to purchase any £20m player from world football and they 'do not fit in, pull up injured' the chances are their valuation will depreciate anyway. See: Davy Klaassen, not that I think we should sell him in any way shape or form but if we were we would be looking at a price depreciation.
Realistically, to take the next step in progressing we need to a) narrow the quality gap between ourselves and the teams above, and b) change this losers mentality that is running through the squad.
As an individual Theo Walcott achieves both a & b. He is better than what we have and his head is more experienced and potentially more driven than some of the players we have here at the moment. He's got a World Cup place to fight for so he won't be filling the numbers.
We don't need someone who could take 2 years to settle into the English game or someone we have to bide our time with, that is Vlasic and Lookman - ready made and in the waiting. We need someone to set the example for those individuals, and at £20m Theo Walcott is an absolute economical steal.