The 21-year-old has suffered a season blighted with injury that originally offered so much promise.
McLean, who has been repeatedly linked with moves south of the border, missed the Buddies' semi-final win over Celtic.
And although he is now back to full fitness, he has not been able to reclaim a starting berth in the Saints boiler room consistently in recent weeks under Danny Lennon.
In fact, McLean has started in just one of the club's last three outings, and was an unused sub in the 0-0 draw with Dundee United at the weekend.
Now the arrival of Estonian trialist Sander Puri, who may win a deal with the Paisley club ahead of Sunday, could put the midfielder's hopes of a place in the cup final 16, never mind a starting jersey, in jeopardy.
He admitted: "To play in the final would represent the biggest game of my career.
"But at the same time I know the gaffer has some big calls to make and that the competition for places will be at its most intense.
"Sander has come in and done really well in training this week so we have probably never been as strong as we are at the moment in the middle areas. But you have got to respond to that.
"I started at Dundee but was left on the bench against United at the weekend.
"So it is up to me to make sure I do as much as I can in training over the remaining few days to convince the gaffer I am worth a start.
"The fact we have come away to St Andrews has been great in that it has allowed us to focus everything on the final, and make sure that we give ourselves the best platform possible to lifting the trophy.
"After missing out on the semi-final I am desperate to play in the final and I will be doing everything I can to make that happen."
While Saints may have earned a psychological advantage over Hearts by beating their Edinburgh opponents the last time the two sides met in Paisley, McLean says that will have no bearing on Sunday.
He said: "I think a cup final takes care of itself. It was great to get that win but Hearts have come back strongly since that game in the SPL.
"They have beaten St Johnstone and drawn with Hibs. The team has responded to interim coach Gary Locke really well.
"They are a big club who will take a huge support to Hampden and that will give their players a massive lift.
"So I don't think the league game will prove a factor."
While Saints' hopes of securing a top-six berth in the SPL look to have gone, McLean has no doubt that becoming the first Paisley side to claim cup silverware since the Scottish Cup winning team of 1987 would make the class of 2013 icons at the club.
He said: "When you come into the ground you see the photos of the 1987 side and it would be great to see our team up there with the trophy as well.
"It's a massive achievement for a provincial side like ourselves to make a cup final and we want to make sure it is a day to remember."