Rashford is on-loan at Barca this season from Manchester United, but was dropped to the bench for the weekend win against Getafe due to being late for a pre-match meeting.
"We know that if Rashford is ready and if his attitude is right then there's a player in there," Shearer told Betfair.
"He was sensational against Newcastle. The second finish in particular was amazing. But you can't be unprofessional. Why should everyone hang around just for you? Why should everyone else be on time but you be late? It's not right.
"I guess if he wants to stay there, particularly only being on-loan, you can't do that when you're at a top club like Barcelona because there'll always be someone to take your place.
"It doesn't matter if you're two minutes late or 20 minutes late. You're late and your teammates have to wait for you. Why should they all be on time and you not?"