The Chelsea loanee is likely to be out until the middle of December after coming off with a hamstring injury in last week's 2-1 defeat to Brighton.
He is likely to miss Villa's next three games, starting with Monday's Premier League trip to West Ham.
"Ross obviously got a hamstring injury against Brighton so he's going to be out for two or three weeks. Nothing more than that, it's not as bad as what we first feared," Smith told a press conference.
"He's done a week already, obviously. We signed Ross because he's a quality player and if you have quality players come out of your team it's going to affect you a little bit.
"Plus we've got nobody similar to Ross in terms of how he plays. So it will be a loss.
"But we won our first two games without Ross as well, so we certainly haven't lost sight of that. We're not going to rush him because we don't want to lose him again."