Harper, 37, admitted players will have to get used to that as the season progresses.
He said: "Yes, especially given the volume of games we have. Sometimes the manager will leave people out for their own benefit to keep them fresh and give them a little rest.
"It's important when you do get the opportunity that you try to take it. The manager has said the league has to be the priority for this club.
"Everyone involved last year, it was an absolutely incredible effort from everybody to finish fifth in a very tough Premier League, and that has to remain the priority.
"But Europe is an opportunity for people who haven't maybe played so much in the league to send a message to the manager - Demba did it very impressively on Monday night - and it gives a few people the opportunity, whatever team the manager picks, to let him know they want to be featuring in the Premier League games."