The 2-0 defeat at McDiarmid Park was United's third successive league loss to Saints, but Souttar insists it will have no bearing on the Celtic Park showpiece on 17 May.
"I think cup finals are always one-off games and not affected by other results," he told the Courier.
"You saw Birmingham beat Arsenal in the League Cup down in England a few years ago, while Wigan beat Manchester City in the FA Cup final as well.
"It is hard to predict the outcome of finals and it comes down to what happens during the 90 minutes.
"So I don't think this will affect us at all. We are a confident bunch and I think we're capable of beating them on our day.
"But I don't think we should dwell on this one performance. We should look forward to the cup final because it's a massive occasion."
Don't forget you can watch all the action from the Scottish Cup final on 17 May on Sky Sports from 14.30