The Tigers have already lost the likes of James Chester and Robbie Brady following their relegation from the top flight last season, while speculation is mounting over the futures of midfielder Tom Huddlestone and strikers Nikica Jelavic and Abel Hernandez among others.
Bruce is desperate to hang on to his biggest stars in order to mount a sustained tilt at promotion, but he admits the matter is out of his hands with the transfer window open until September 1.
Speaking after City’s 2-1 victory over Fulham, Bruce said: “If we’ve got what we’ve got now then I’m very, very confident that we’ll mount a challenge.
“If we lose another four or five then we’re goosed, that’s my fear.
“That’s being brutally honest and that’s what we need to guard against. I hope it doesn’t come down to the last two or three days (of the transfer window) when we lose these players because then it becomes very, very difficult (to replace them).”
Bruce’s men continued their unbeaten start to the Championship season as Sone Aluko’s close-range strike four minutes from time edged them to victory over the Cottagers.
However, it was a hard-fought win as, despite dominating the first half, they were pegged back when former Hull midfielder Tom Cairney’s 69th-minute strike cancelled out Ahmed Elmohamady’s headed opener.
“To be fair to Fulham they played better in the second half,” said Bruce.
“So all credit to Fulham, but we made a couple of mistakes for the goal which we’ve got to eradicate. It was a great finish by Tom but we made three mistakes in the build-up to it.”
Chief among the errors was another gaffe from goalkeeper Allan McGregor, whose howler in gifting Wolves a goal over the weekend was widely highlighted.
The Scotland international once again dropped the ball from a cross, and although Hull were able to ride the first wave of attack they were unable to regroup and were out of position by the time Cairney struck with a wonderful left-foot strike.
“We’ve got to stop that,” added Bruce. “If we want to know how difficult the Championship is we’ve just witnessed it.
“We showed a bit of resilience, and we didn’t really play well in the second half after the first-half dominance. I was really pleased the way we played first half.
“We’re unbeaten since we’ve been back, we’ve gone four games now, so we’re starting to get a little bit of confidence and that can only help the situation.”
Fulham have conceded late goals in all three of their Championship matches to date and it is driving manager Kit Symons to distraction.
“(We had a) poor first half and conceded a very poor goal but weren’t really in it, didn’t play well at all,” he said. “Second half, we really upped our game; played on the front foot and took the game to Hull and scored a very good goal through Tom, and we’re in the ascendancy then.
“To then go and lose it from that position is hard to take but it’s only three games in and I’m sick of saying the same things already.
“It can’t be down to fitness, it must just be down to concentration, not being street-wise enough. It’s costing us dearly and it needs to change.”