McShane has not featured for Hull since a 1-0 Premier League defeat at Burnley last month.
The Republic of Ireland international took to Twitter last week to reveal he was training with the club's under-21 side, a move that Bruce has not taken kindly to.
"There's been a lot talked about Paul McShane, it's quite simple," said Bruce, who would not reveal the nature of the fallout between the two.
"I've heard a lot about nonsense on Twitter.
"He knows why he's training with the under-21 squad at the moment. He's been quite quick to reveal that he is in there.
"I hope that he comes out and tells the reason why.
"If he apologises to myself and the squad then he will come back into the fold. It's entirely up to Paul."
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