I don't credit Emery much at all. There was an injury crisis, and someone needed to go on to play left wing back. 17-year old Saka was happy just to play, didn't mind he was out of position, and took his chance. The rest is history, because he kept taking his chances. Contrast to Maitland-Niles, who probably had far greater talent at the same age, but all reports say was resistant to playing right wing-back because he thought he was an attacking midfielder. Saka has had some fortune (Arsenal injuries got him a chance before he would otherwise been brought into the first team; he's my favorite player, but he didn't earn his first England call-up, it was a race between England and Nigeria to see who would call him up first). But every chance he got, he seized with both hands. 95% of it is on Saka; Emery, and especially Southgate and Arteta, just got lucky.