Alexis has been playing extremely well of late, there is no doubt. I'd actually go as far as to say he will get even better still with more confidence and playing time. However, the fact remains Alexis is not the ideal player for that wing, on this team.
Barca's midfield is creative and outstanding at game control, however Barca's midfield is not a scoring midfield. None of the players are proficient at scoring from a midfield position, save Fabregas. With this in mind, Barca needs scoring forwards much more than they need creative and play building forwards.
It would be ideal if they had forwards that could do both, like the genius Messi does, and like Neymar is perhaps close to becoming, however Sanchez is not, at this point.
His nice goal scoring form of late is perhaps a sign of changes to come, however I still feel he is not a natural goal scorer, and perhaps he'll never become one.
This is why Pedro fits so well. He is a scoring forward, and even though he is not great nor world class, merely good, he still is able to shine at Barca.
Tello hasn't really shown to be in any of the two categories. He is not particularly a creative forward, even though he has shown to be a capable crosser; nor has he shown to be a particularly adept scorer. IMO Tello would be better suited to an open style league like the EPL or Seria A, where he can better exploit his speed and crossing ability. There is where he should go, be it sold or loaned, and then perhaps we can find if he ever becomes a better scorer.