Walcott hadn't played a match above Championship level, and was clearly judged at the tournament to not be ready to play.
TAA has excelled in two Champions League quarter finals against one of the best young attackers in the world, and possibly the best team in Europe. Plus 23 PL appearances, not a huge number but infinitely better than nothing.
I've seen nothing to suggest that he'd be unready to step up if needed. I personally feel he won't go because England have 3 other good rightbacks, two of which seem able to slot into a back 3 which is a huge advantage in a squad and tournament where versatility is a key quality.
If he goes, he'll have earned it.