Barkley was given a gentle ticking off by Smith for his reaction at being withdrawn against Arsenal last month with the Scouser ordered to pay for a new bottle bag after booting it in disappointment.
Smith said: "He was fine. As I do with all players when I change a team, I explain my reasons to them. I explained it to both Ross and Douglas (Luiz) and, while players are never happy coming out of a team, they understood my reasons because they were the conversations I had. I just felt that it was time to change it for that game.
"But he is up to speed. He is getting the training minutes that he needs now. I said all along that unfortunately for him he was having to get his fitness up during games.
"We had a period of games where we had no training and he was playing. So he is up to speed. I decided to change it at Leeds knowing the way Leeds played.
"I didn't think Ross and Douglas had probably played to their standards that they have been in the previous game against Leicester and, I thought, it was time to have a little change
"But he is fit and available and in contention to start against Sheffield United."
Smith added: "In terms of what I've found of his character, he's as professional as I've came across. His work on the training ground and in the gym; he's probably one of the last to leave each day when he's here at Bodymoor Heath and he's got a drive to get better all of the time, which is really pleasing.
"He's certainly encompassed what we're all about and has fitted into what Aston Villa is about as well."