IN GENERAL, not just with Rasmus, not getting enough service it's not the perfect excuse, because at some point you have the ball and you have to not only try to do sthg. on your own, you have to actually produce sthg. more often than not.
Also pressuring, even if it's useful per se, actually adds another dimension when the player actually steals the ball regularly with such actions.
For instance Erling have been quite bad prior to this last match, not only because City didn't provide enough service (or play to his strengths, BTW they rarely did that since his arrival a la Dortmund days), but because he mostly was too estatic, a bit to casual with the little service he got and not productive enough.
In his last game he didn't score, but he found better pockets of space, he was more calm with the ball, he was more agressive and intelligent with his movement and he played a very good game without scoring and even assisted.
As a side note, City as a team that usually has the ball a lot, many times makes people think that the strikers have tons of chances galore because they are focus on that, that they play to their strengths and it's not the entirely the case. Traditionally in City chances arrive almost for anyone as a product of volume, of the pressure that Pep puts on players to give it all and do specific tasks in specific matches, but barely because they play to the strengths not only of some player, of even some role. City is weird in such regard, people it's too simplistic regarding them.
And finally, if he actually is a pure goalscorer, more like a poacher, or it's what best suits him...any club that pays 70 millions for him, even with the caveat and responsabilities any player must embrace written at the beggining, they HAVE to at least try to play to the player strengths (Pep it's Pep, an anomaly), if not, why the feck you buy Chicarito expecting to dribble his way to score, or Palermo and not constantly serving balls for him to head, etc. etc.
This doesn't happen in United with many players, many roles, because since time it does not function properly as a team and asks to many times things to some players that to the very least will requiere an adaptation or would never happen because they just can't be that player.