Ryan Stevenson is pushing for more involvement after making his comeback from a knee ligament injury in the 71st minute of last weekend's goalless draw with Dundee United.
However, Locke could not give him game time for the Under-20s in midweek as he did not want to play the midfielder on an artificial surface.
Midfielder Jamie Hamill is relishing the crucial nature of the clash.
Hearts are looking to cut the gap at the bottom of the table to six points after St Mirren's poor start to the campaign gave impetus to their bid to overturn a 15-point penalty and avoid relegation.
Saints boss Danny Lennon saw the pressure on himself slightly eased when his team collected their second point of the campaign, against Aberdeen on Monday.
But Hamill is revelling in the demanding nature of the game and his club's predicament.
Hamill said: "I'm looking forward to the game first and foremost. It's going to be a big game considering the points difference but it's one we are looking forward to.
"I think we have been under pressure from day one so I would say it's ourselves (who are under most pressure).
"We are putting pressure on ourselves to try to boost up the league. And that's good because I think we have been playing really well."
Lennon will have to put his faith in defender Lee Mair.
Marc McAusland (ankle) is still out and Mair should retain his place from Monday's draw, despite being told in the summer he was free to find a new club.
Their only other absentee is long-term injury victim David Barron (ankle), while left-back Danny Grainger returns from suspension.
"Every game is a cup final for us now,'' admitted midfielder John McGinn. "We are down there and need to get ourselves out this hole we have got ourselves into.
"But I still definitely prefer to be in our position than theirs because we are higher up in the table.
"It's not nice because I've got a lot of friends at Hearts from my time with the Scotland set-up. It's not nice to see them going through administration and people losing their jobs.
"But the team have done really well. Now we just need to hope we can get the better of them."