The 29-year-old struggled to establish himself in his first season at Meadow Lane last year, scoring just once as injury disrupted his campaign.
But Allen believes the forward has worked hard to prove to him that he can be a hit with the Magpies after losing a stone in weight since May.
"I told him in the summer he had to get fitter and if he did then I would have him back involved. That's what he's done," Allen told the Nottingham Post.
"He needed to get in better shape and he has done it. When I told him I was taking him off the transfer list he had a great big smile on his face because he wants to do it here.
"He has showed me how much he wants to be a success by losing so much weight and working so hard to get fitter."