It's probably too soon for Barcelona to evaluate Marcus Rashford's performance and think about sending him back to Manchester United, you're right.  Although Rashford hasn't exactly lit up the globe in his three games with Barcelona, the team's management appears to have faith in his abilities.  Barcelona has reassured Rashford's agency that they believe the player will recover and become a good player, according to Spanish football commentator Guillem Balague.  This implies that the team is prepared to allow Rashford additional time to get used to his surroundings and establish himself.

https://fcbarcelonalatestnews.com/barcelona-are-considering-sending-marcus-rashford-back-to-manchester-united-167841/

Given the current situation, it seems that Barcelona is willing to give Rashford more time to settle in and prove himself. However, if he continues to struggle, the club might reconsider their decision to sign him on loan