Hearts have taken eight points off their 15-point penalty for going into administration ahead of the visit of the Paisley side, who collected only their second point of the season on Monday night in a 1-1 draw with Aberdeen.
However, Locke saw some signs of second-bottom Saints improving after Steven Thompson hit the opener against the Dons.
Locke said: "Psychologically it would be a huge win but we watched them the other night and, once they got a goal ahead, they looked as if their confidence was coming back.
"They have a lot of good players, the big boy Thompson up front is a handful, (Paul) McGowan just off him is a good player.
"I know they have not got off to the best of starts but they are a good side and we know how tough it's going to be."
Locke dismissed suggestions that this was the biggest game of the season so far for Hearts and repeated words from assistant Billy Brown to highlight his point.
"They are all big games," he said.
"Since the season started every game is a cup final for us because of the situation we are in.
"Obviously St Mirren are the team above us but we are going into this game no differently from any other.
"It's a home game, we are going to try and win it, but it's just as important as the next five or six games coming up.
"Billy hit the nail on the head the other day - we are not going to avoid relegation after eight or nine games.
"We have 38 games to eat into the points deduction and that's what we aim to do."
And Locke does not feel the nature of the bottom-of-the-table clash could result in a nervy atmosphere among his young side.
"I don't think so," he said.
"I can't talk about St Mirren, I can only talk about Hearts. But if the fans come and get behind us as I know they will and as they have done every home game this season, we will just tell the players what we have done before every game - express yourself, go and play and play like we have been playing.
"And hopefully that will get us the result we are looking for."
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