David Martindale heaped praise on Max Stryjek after the goalkeeper made two "superb" penalty shoot-out saves to send Livingston into the Premier Sports Cup quarter-finals.
The Polish goalkeeper was the villain last weekend when he allowed Jack Mackenzie's injury-time shot to squirm from his grasp and over the line in the 2-1 defeat to Aberdeen.
But the 25-year-old was the Lions' hero on this occasion as his stops from Cameron MacPherson and Kyle McAllister earned his side a 4-3 victory on penalties.
Alan Forrest's first-half opener for Livi had been cancelled out late on by Conor McCarthy's equaliser, but the home team prevailed in the end to move a step closer to emulating last season's progress to the final of the competition.
Martindale said: "I thought the big man was brilliant. I thought his two saves were superb, especially the second one to win us the game from McAllister.
"He did cost us three points last week but I still never thought about replacing him this week.
"He's a totally different player this season; he has really become a leader for us this season.
"I wanted more from him and since pre-season he's been brilliant on and off the park.
"He has stepped up, he has become one of the main players and is now one of the first names on the team-sheet."
Livi thought they had won the tie in extra-time when Nicky Devlin followed in an Ayo Obileye header that ricocheted off the post.
And, despite the officials' ruling otherwise, Martindale claimed even some St Mirren players confessed the ball had crossed the line.
The Lions manager added: "I spoke to two St Mirren players after the game and they said it was a yard over the line.
"I have to be diplomatic as no official goes out to make a mistake.
"But, if I was sitting here after we had lost on penalties then I might have a different attitude."
Meanwhile, St Mirren counterpart Jim Goodwin lamented his side's inability to win the game in regular time, before injuries and fitness left them happy to take on the "lottery" of penalty kicks.
He said: "I didn't think it was the greatest spectacle. You know Davie's team are always going to be hard working and well organised.
"We spoke about the counter-attack, which is where the goal comes from. We lost possession cheaply. We shouldn't have let Forrest come back onto his stronger foot.
"But I knew the boys wouldn't give up. We looked as if we would go on to get the winner.
"It's a lottery [penalties]. You win some, you lose some.
"I can't fault the guys who stepped up, they have the courage to put themselves out there and it's just unfortunate they are on the receiving end of a defeat."
