If he's as good as everyone says he is then he could probably be ready for us next summer. If he's good enough he's old enough, and the best ones are old enough very early. Varane went from Lens straight to Real Madrid and never missed a beat. I think it's good we're in early for Saliba, but it hinges on him being extraordinary of course.
Agree that it's hard to feel excited about him right now though when we're doing nothing to improve our chances this season. Wenger would have been lynched if he'd made a deal like this while ignoring the gaping holes in the starting eleven.