OK, so presuming that the stats on whoscored.com are correct, and excluding the games against Real Madrid, PSG and Bayern, their stats are the following (I hope I haven't miscalculated anything):
Tello: 1386 minutes, 5 assists, 8 goals
Sanchez: 2069 minutes, 11 assists, 10 goals
...which makes for an average of 277 minutes for an assist and 173 minutes for a goal for Tello, and 188 minutes for an assist and 206 minutes for a goal for Sanchez. There you go.
First of all, stats are just stats and can be very misleading. As far as I'm concerned, Tello has been the better player just by observing both of them throughout the season. But if you insist on basing your argument on stats, fine. Judging by those average stats, your claim that Sanchez has the better stats is not entirely true. He only has the better assist stats. We might also take into consideration the fact that Alexis plays in the starting 11 more often than Tello, so it's easier for Alexis to score or assist if he's surrounded by better teammates. But while we're at it, a forward's primary job is to score, and Tello is much better than Alexis at that.