They were fined a six-figure sum after making the Premier League aware they did not have international clearance for the South Korean.
Hull boss Bruce, who signed Ji when he was Sunderland manager in the summer of 2011, told a news conference on Friday: "That's pretty bizarre stuff that it's only surfaced now and he's played in four games.
"They should be in serious trouble, I would have thought."
Asked if he would expect a club fielding an ineligible player to be handed a points deduction, Bruce replied: "It's the first I have heard of it and it will be interesting, I would have thought so though."
Sunderland consider the matter closed and are confident they fulfilled their responsibilities when the administrative error came to light, and insist there is no ongoing investigation amid speculation that the relegation-threatened club could yet be docked points.
Ji initially joined the Premier League side from Chunnam Dragons for £2m in June 2011 during Bruce's spell in charge, but started only six senior games for the club.
He had a loan spell with Augsburg last season and joined the German club on a permanent deal in January.
Sunderland currently lie in 19th place in the table, four points adrift of safety with just eight games to play.