He says players who impress during the week before a game get picked - with boss Alex McLeish having no favourites when it comes to selection.
Grant told Villa's website: "Andreas came on against Wigan because he deserved to. One of the reasons we put him out on loan for a month was because in pre-season training we thought he was excellent.
"We brought him back with the proviso that he would be involved with us and he's thoroughly deserved that. Nathan has been fantastic too.
"What we tell the players is every day they have to produce the goods and not just on a Saturday.
"They need to show it in training and train exceptionally well because that is what we are watching all of the time."