Bolo Zenden joined Anton Ferdinand and John Mensah on the treatment table when he injured his hamstring on Wednesday, thereby lengthening an absentee list that also includes suspended duo Phil Bardsley and Kieran Richardson.
Michael Turner, Paulo Da Silva and Nyron Nosworthy are currently Sunderland's only available defenders, but Bruce is desperately hoping that McCartney will be passed fit to complete a patched-up back four tomorrow morning.
The Northern Irishman missed last weekend's defeat at Fulham with a hamstring injury, and is yet to return to full training.
If he fails to recover in time for Pompey's visit to the Stadium of Light, Bruce will be forced to drop skipper Lorik Cana into the back four, a move that would seriously weaken his side's midfield.
It's pretty serious stuff,
said Bruce, who has watched the Black Cats win just one of their last six matches.
Mensah won't play, and neither will (Lee) Cattermole, (Craig) Gordon, Ferdinand and Zenden. Bardsley and Richardson are suspended, and there's also a doubt about McCartney. We are down to the bare bones The key issue is whether McCartney is fit. If he is, then with Da Silva, Turner and Nosworthy all in the equation as well, I think we'd be okay. I think the McCartney situation is the key. It will be a massive plus if he can come through.
Nevertheless, the regularity with which Sunderland's players are succumbing to muscular problems is becoming a major source of concern to Bruce.
Mensah has made just two league starts since joining from Lyon in the summer, with a succession of pulls and strains forcing him to spend weeks at a time on the sidelines.
McCartney has also suffered a series of hamstring niggles since rejoining Sunderland from West Ham in August 2008, while this week's injury was Zenden's second in the six weeks he has been a Black Cats player.
It's a huge frustration, admitted Bruce. When John Mensah pulled up last week, he was distraught.
He's left his family and everyone else to come here to play football, but he keeps on picking up injuries.
It's been difficult to keep him on the pitch, and the same applies to George. He's had a horrific run of injuries.
There's nothing worse than having that sort of scenario, where they come back and get injured again. Bolo's only been here six weeks, but he's already pulled up twice as well. It's frustrating for me, and for them.
Having watched Sunderland's reserve team on a number of occasions in recent weeks, Bruce has decided that there is not an emerging defender capable of being fasttracked into the first team.
However, the Black Cats boss has been impressed by 18-year-old striker Ryan Noble, and will name the Sunderland- born youngster on the substitutes' bench tomorrow.
Noble has scored 32 goals in 23 starts for the reserves and Academy teams this season, and underlined his potential with a hat-trick in this week's FA Youth Cup win over Sheffield Wednesday.
He travelled to Wigan with the rest of the senior squad two weeks ago, and while he failed to make the matchday 18 on that occasion, he will be promoted to the bench for tomorrow's game with Pompey.
It's a certainty that he'll be involved, said Bruce. It's a quite exceptional record that he's got so far. It's amazing, and it's not in a lot of games either.
He's still got a hell of a lot of growing to do, but we took him to Wigan and he took massive delight in telling me that he was a Sunderland fan when he was getting on the team bus. That's fantastic.
I've seen him score two hat-tricks and he has exceptional pace and a fantastic finish.
It's a massive ask, but who knows He's lightning quick, so I don't think he'll have to rely too much on his physicality.
Noble's emergence will strengthen Bruce's resolve to part company with Roy O'- Donovan and David Healy next month, and the Sunderland boss will also assess Ferdinand's future amid continued speculation linking the London-born centre-half with Tottenham.
Bruce has ruled out allowing Ferdinand to leave on a loan basis, but tellingly stopped short of issuing a hands-off warning to Harry Redknapp.
It's come from the summer when Harry came in on deadline day, said the Sunderland boss.
He had a problem at centre- back and asked if Ferdinand could come on loan.
The simple answer was, No, I'm not interested in anything like that'. Whether that is still in Harry's mindset I don't know, but he certainly won't be going anywhere on loan.