Relative to other players: Defenders overtook him. I haven't seen him directly alongside Sane and Sterling but when you see City on the counter-attack, few keep pace with Sane and Sterling. 2nd half at the Emirates, Arsenal were just playing on the counter-attack and picking off Spurs high line. They had several counter-attacks with sanchez to the fore. More or less every time defenders just ate into the advantage he had. My impression was that when Sanchez was at Udinese and barcelona, and when first at Arsenal, very few defenders could keep pace with sanchez, but that I think has changed, and given his age I think it will get worse and takes away some of the effectiveness in his game. He's still a technically gifted player, but I prefer all our forwards over him. I can't give you a metric other than general observations that he doesn't seem able to beat defenders one for one any more and when played through into space behind a defence, defenders catch him up.
I remember years back, there was some kind of competition between quick players who were timed in sprints, and Curle came out near the top. We don't have anything like that so I just have to observe how they do one on one with players, and my observation is that what was once a key asset of Sanchez's game has gone. I know it's vague.