What was the point of going to Barcelona to sit on the bench?
Barcelona have one of the best attacks in the world at the moment and only PSG could be better.
Olmo is not a competitor for Rashford, rather Lewandowski, who is apparently looking for a replacement, but last season Lewandowski scored 42 goals and that's a great result, and Rashford scored only 11 goals for United and Aston Villa last season.
Taking into account the salary of 12 million euros a year at Rashford the meaning of his rent for Barcelona is still a mystery and besides for Rashford it would be more useful to play another season in Aston Villa, which got into the European Cup and is quite a competition to many top clubs in Europe.
I think Rashford will not only fill the bench, I'm sure flick already has a plan to utilize the potential of this player and besides, as you said Lewandowski is old enough so he needs a substitute if the match is not too important then Rashford will be used to replace Lewa or Rafhina is shifted to number 9 and Rashford plays in his original position. However, Rashford has joined so Barcelona must use him.