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Du Plessis was speaking after his side's huge 211-run defeat to England at Lord's on Sunday.
Duminy has failed to cross 40 runs in his last eight innings. He managed just 15 and two runs at the Lord's-something which forced du Plessis to admit that Duminy is not doing enough to maintain his place in the side.
With du Plessis set to return to the side after missing the opening Test due to the birth of his first child, one option for him would be to take the place of Duminy.
However, the skipper insisted that the Proteas would consider a few other things before coming to any conclusion.
Du Plessis said that Duminy is a crucial part of their current senior team and that he is well aware of the fact that he needs to score runs to keep his place in the national side.
"JP will be the first guy to say he knows he needs to score runs for this team and it's no different for anyone else," Sport24 quoted du Plessis as saying.
"He is desperate to do well as anyone in that position. You want to try and score runs every time you go out there. He knows at the end of the day that it's about runs," he added.
Du Plessis further insisted that Duminy always keeps team's interest above himself and is always desperate to do well at that position than anyone else.
"He understands that if it has to come to a position where there is someone else that needs to be looked at... he will be the first to acknowledge [that].He is a crucial part of our senior player group and he will always put the team interests above himself so yes, he will be the first to acknowledge that," du Plessis said.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)