Suspended midfielders John Rankin and Willo Flood have joined long-term injury victims Johnny Russell and Gavin Gunning on the sidelines ahead of the visit of SPL bottom club Dundee.
And captain Jon Daly and defender Brian McLean are both struggling to make Sunday's Tannadice clash.
McNamara knows the situation is far from ideal but he urged whoever is handed the chance to seize the moment by staking a claim for a regular starting slot.
He said: "It's not looking great squad-wise for the derby.
"A couple of youngsters could be thrown in because of that.
"I would say Jon is more doubtful at the moment than Brian because of his groin.
"Brian came off against St Mirren last weekend with a wee tweak to his hamstring.
"So hopefully he might have a chance of making Sunday but it will be a last-minute decision.
"It's about getting the right balance with the squad that's important.
"We seem to be getting hit in certain areas of the team.
"We are looking to guys to do a job when they have not had a lot of football, not just in the last few weeks but in a number of months.
"I always look on the positive side of things though.
"This is a chance for players to go and show me they deserve a place in the team."
McNamara savoured victory in his first Dundee derby when his team defeated the Dark Blues 2-1 earlier this month to set up a Scottish semi-final against Celtic.
The Tangerines boss is now preparing to welcome John Brown's Dundee to their own patch with his sights set firmly on a quickfire double.
McNamara said: "It would be good to make it two wins in a row.
"That's what we're aiming to do and it would mean a lot to do so in a short space of time.
"We know it's going to be very tough as Dundee will be fired up to try and get the three points.
"But we have to concentrate on ourselves and try to do the things we know we're capable of.
"I'm sure it will be a typical derby game with lots of commitment because we know there's a lot at stake for both clubs as always."
McNamara wants the United fans to play their part as they get ready to outnumber the visiting support by three to one on the day.
He said: "The fans have been great since we came in.
"We know the derby means to much to them and they will get right behind us from the start.
"It could help us that we're going to have more supporters there than Dundee.
"But we also have to give them something to shout about."