Montgomery has questioned how Newcastle have approached this transfer window.
"I would question Newcastle's lack of aggression in the transfer market," he told BBC Radio Newcastle.
"In the modern day, you have to be aggressive when you want to sign a player. When you identify a player and he's your man, then you have to get to him, get to his face, sit with him and tell him he's the best thing since sliced bread.
"Or alternatively, get him on a private jet, get him to that stadium and show him the videos of the fans when Newcastle won the cup to show the passion of the club. That's what sells the club, and to do that, you have to be aggressive. And sadly, I just don't see that with Newcastle at the moment.
"They messed up signing young James Trafford for over a year when they could have easily finalised something. And now you have a situation whereby Newcastle could lose arguably one of the best strikers in Europe, if not the world.
"That is really disheartening for the fans who believed that they were going to go places."