Rashford is on a season-long loan at Barca from Manchester United, with the deal carrying a €30m permanent option.
The England attacker has impressed and after victory at Celta Vigo, he said last night: "I'm doing very well, I'm happy here at Barça. We have to stick to the plan as (coach Hansi) Flick has asked.
"I feel great. My first assist today was probably my best assist here."
However, Carreras believes Rashford's fee can be spent better elsewhere, particularly given Barca's current financial situation.
He said on Carrusel, "If you want to go for a top striker who's going to cost you what he's going to cost, maybe Rashford isn't an option, or you can't do both deals.
"He shouldn't be Barça's number one priority signing, because we're talking about 30 million euros, which is a lot of money given the current situation."