MacPherson brought in Chris Innes to move to a three-man central defence to cope with the poor Fir Park pitch and Hearts' physical presence and the tactics paid off, but he gushed praise on his players.
"It means nothing to me, it means everything to the players," he said.
"It's not about me, it's about the players. I thought they were a credit, every single one of them stood up to the plate.
"Questions have been asked of them in the last couple of weeks but they have certainly answered them. I thought we thoroughly deserved to win."
Mehmet's controlled top-corner strike took his tally in the tournament to seven goals and MacPherson was full of admiration for the striker's finishing prowess.
The Saints boss said: "He is capable of that, in the cups, it's just the league he can't do it! No, he is technically a very good striker.
"He missed a very good chance on Saturday against Hibs when he probably would have been better putting the front of his foot through it, but he always tries to show composure and pass it in and he has done it again tonight. It was a fantastic finish."
Jim Jefferies, who led Hearts to two cup finals in his previous spell as manager, feels he needs time to instil an attacking ethos in the team.
However, he felt there was little between the sides and thought there was significant improvement from his first game back in charge, another 1-0 defeat on Saturday against St Johnstone.
"We played a million times better," Jefferies said.
"You could see there was a willingness to get forward more than they are used to. It will take time to get them into our way of thinking.
"Both teams deserve a lot of credit for trying to make a game of it on an unbelievable pitch.
"It was a hard-fought game, it always looked like one goal or something special was going to win it. They just got a great goal and we didn't."
Source: Team Talk