The Tannadice side take on Neil Lennon's Hoops at Hampden and had kept their fingers crossed that the former Scotland Under-21 attacker would be fit enough to take a place on the bench after suffering a fractured fibula against Inverness at the end of February. But, McNamara confirmed Russell has not been given the green light to resume playing duties.
He said: "We thought Johnny had a wee chance of progressing this week and maybe get a place on the bench so he could come on. But he has broken down doing some stuff in training and it is clear the game has come too soon for him."
The news will come as a blow to Russell, but his team-mate Stuart Armstrong says the rest of the United squad are thriving on the new positive atmosphere introduced by the recent appointment of McNamara as boss.
The former Partick Thistle manager replaced Peter Houston in February and succeeded in guiding the Arabs into the top six of the Clydesdale Bank Premier League with Rory Boulding's 93rd-minute winner against Aberdeen last weekend.
"It was the perfect way to prepare for Celtic in a big semi-final," said the midfielder. "It's given us a lot of our belief back. Now we're hoping to take that into Sunday. There's a confidence about the boys that I have noticed since Jackie came in. Everyone has been given a fresh start. Was a change of leadership needed at that point? Perhaps.
"I've got a lot of respect for Peter Houston - he gave me my chance here, so I've only got good things to say about him. But maybe a fresh start was what the club needed and now everyone is firing off that."
As for McNamara, he will take nothing for granted at Hampden, in spite of their recent miserable Mount Florida record. The Glasgow giants have lost five of their nine ties at the National Stadium under Lennon - including their last three. With five SPL games still to play, they have also dropped 30 points already this term, nine more than the whole of last season.
But McNamara warned: "It can work both ways. I'm sure those recent results will be in the back of their minds. But we can't look too much into things like that or other stats. The most important thing for us is that the players go down there, don't freeze, enjoy the occasion and play their normal game.
"The games that Celtic have slipped up on this year, they have had the distraction of the Champions League to concentrate on. But it looks like they will win the league comfortably and this is an extra opportunity for them to wrap up the double. But it's important that we play as well as we can and hopefully cause them problems."
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- Peter Houston
- Clydesdale Bank Premier League