Professional Footballers Association chief executive Gordon Taylor said: "Whilst it may make good copy and profits for the book publishers and the writer, it puts Roy in a bad position.
"He could face action from the FA, if not possibly legal action from Alfie Haaland. We tried at the time to calm things down between the two and we've had too many player versus player cases. We don't want that to happen again, but this has just opened up old wounds again at an unfortunate time."
"I think Roy has been ill-advised and his comments are ill-judged. Roy hardly needs the money and I just think he could do without all this."
Taylor added:
"We're talking about two clubs in the same city and both in the Premiership now. There's a lot of rivalry between the fans and that's good, but it can boil over and we don't want that to happen here."
"I can't believe he's been advised to do this bearing in mind the events of the summer. Roy is a committed player and a forthright player, but following what happened in the summer I just don't think he needed this. This puts him under pressure publicly when really he needed a quieter time. It would have been better for him to get his head down and focus on playing for Manchester United."
FA
A statement on the FA's website said: "Having read the comments we would like to see the book in its full context before making any decision. However we clearly would never expect any one individual to set out to physically hurt an opponent at any level of the game."