If he is given the opportunity to play more, I'm sure he will give his best for Barcelona, but the question is whether Hansi Flick will give Rashford a starting spot because, as far as I know, Barcelona's front line is already filled with Lamine Yamal, Rapinha, and Lewandowski. But I think Rashford is only used as a rotation option when Barcelona faces average clubs in the La Liga competition later. Well, when Rashford is given such an opportunity, it should be an opportunity for him to show his best so that management is interested in making him permanent.
If Rashford did not perform well at Manchester United and West Ham, with me, there is no reason to believe that he will contribute his best for Barcelona. If this deal does not cost Barcelona too much money, it's acceptable as Rashford is only one ace under their sleeves and nothing more. They already have many world class attacking players to compete for titles in a new season.
They need to prepare a good striker to replace Lewandowski and Rashford is not a striker or can not play too well at this position.