Mignolet joined the Black Cats at the start of the summer on a five-year-deal as back-up to Gordon, but found himself thrust straight into first-team action when the Scotland international was sidelined for three months after fracturing an elbow in training.
Bruce admits he almost brought another goalkeeper in to compensate for the injury to Gordon, fearing it would be too much for the youngster to be put straight into the firing line.
But after a win against Manchester City and draws against Arsenal and Liverpool, the former Wigan boss couldn't be happier with Mignolet's first few months in the Premier League.
"He's fantastic," said Bruce. "He's exceeded expectations without question. I thought long and hard about putting him in, I thought it'd be so unfair on the lad coming from Belgium, we did had a look at David James.
"When we threw him in at the deep end, though, he swam. He's got butterfly, front crawl and breaststroke all at once, the kid's done great. The best thing I can say about him is that we've not missed Craig, who is back, and it's a big compliment.
"You maybe think he's not ready, maybe he could have done better with one of the goals against Birmingham, but I've stuck with him and since the opening day he's grown in stature. The goals that have gone past him are a horrible deflection at Wigan and crazy ones against Liverpool and Arsenal."
The Black Cats face Manchester United at the Stadium of Light tomorrow and, while the game might come too early for £9m man Gordon, Bruce has a big decision to make whether he sticks with Mignolet or puts his number one straight back in.
"It's great to see Craig back, but Mignolet's been magnificent," said Bruce. "It's a decision you hope you get right.
"Mignolet has been fantastic coming in from the first division in Belgium. He's got the jersey and I've got to make a decision.
"Craig's fit and he's joined in the last week. He's back in the Scotland squad, and it will give him a bit of a boost to be on the international scene with two big games.
"If I'm being brutally honest, it's a little bit early for him and I wouldn't expect him to play for Scotland in those games with one reserve game and a week's training after being out for three months.
"It'll be good for him to be round the international squad, their manager has pointed that out and I've got no problem with that. We wish him well."
Meanwhile, while Wayne Rooney sat out Wednesday's Champions League win over Valencia with an ankle injury and is doubtful for the trip to Wearside, Bruce claims he wouldn't be surprised to see the England striker on the team sheet.
The former Manchester United skipper also believes Sir Alex Ferguson is the right man to get the 24-year-old's career back on track following allegations over his private life.
"Is he out for three weeks I don't know whether he is or whether Sir Alex is playing one of his games," said Bruce. "Make no mistake, if there's one person who will look after him better than anybody else, it's Sir Alex.
"It's been a tough time for the kid and it's possibly the reason why what happened in the World Cup happened if we're honest. He's a fabulous player with a strong mental character and he'll need that.
"His (Ferguson's) biggest strength is his protection of his players. Don't get me wrong, Sir Alex will put you right if you step out of line, but I remember years ago him picking on Jimmy Hill to protect (Eric) Cantona after the stamp.
"It changed the whole weekend to take the attention from Cantona. He'll know the best way forward for Wayne, that's his big strength."