The Tannadice boss has "just about" chosen his team to take on St Johnstone in Saturday's William Hill Scottish Cup final.
But he admits he faces the most difficult day of his short managerial career on Friday when he informs an unlucky group of players that they are not in his starting line-up for the Celtic Park clash.
McNamara suffered that same agony 13 years ago when O'Neill left him out of the side that started against Hibernian in the 2001 final - only discovering the news 90 minutes before kick-off.
The Hoops midfielder was given a reprieve when Lubo Moravcik limped off after barely a quarter of an hour but made the most of his chance by firing the first goal in a 3-0 win.
However, the experience of being dropped at the last minute has stayed with the United boss.
"Telling the guys who are not in the team that they won't be playing is going to be the hardest part of this week," admitted McNamara. "Letting people down is the horrible part of the job. Unfortunately it has to be done.
"I've experienced it myself as a player when I was left out of the starting XI of the 2001 final. Martin just read out the team an hour an a half before the game and I wasn't in it. That was it. There is never an easy way but that was his method.
"Thankfully I was able to come on during the match and scored the first goal.
"But everybody is different. I think the best way to let a player down is to communicate with them. I know they won't like being told they are not playing but it is better to be honest and explain why.
"I'm just doing things the way that I would like to be treated.
"I've already had to do it in the semi-final when I had to tell a few lads they wouldn't be starting. Again this week, it's a decision I have to make. If it is right or wrong, then it is my head on the block."
McNamara's preparations for the final were interrupted last week when Blackpool chairman Karl Oyston contacted the Arabs looking to poach their boss.
Tannadice chairman Steven Thompson rejected the Seasiders' offer out of hand while McNamara declined to kick up a fuss as he pledged his commitment to the Tangerines.
But even after seeing Gary Locke and Danny Lennon lose their lose their positions at Hearts and St Mirren this week despite doing decent jobs, he insists he has no regrets about his show of loyalty.
"It's not a gamble for me because I've not made any decision," he said. "There was nothing to talk about there. The chairman called me to tell me there had been an offer but it had been turned down, so that was it.
"There was no job offer, no change. My focus has always been here and preparing for this game.
"I feel for Danny and Gary because they have both done very good jobs and I'm a wee bit disappointed with the way they have been treated.
"But that's the business we are in. You are popular one minute, then the next they want you out the door. That's why I never get carried away when things go well and I don't get too down when things go bad."
United dished out a four goal drubbing when they first took on St Johnstone this season back in August. Since then, though, they have shipped seven and failed to score any as they suffered three successive defeats to the Perth side.
But McNamara is not worried, saying: "Those results will have no bearing on the game. If we had beaten them four times, or the other way about, I would not change anything.
"It's a one-off game. They are a hard-working team and have a talisman up front in Stevie May, which we need to be wary of. But this is a big game and we will be ready for it."