"We knew he wouldn't be able to train until two or three weeks into pre-season," Keegan told Newcastle's official website.
"Then it will be light with a view to him coming into full-time training probably in the next four to five weeks depending on how he feels when we start to push him on.
"But I think what we will get now is no more problems with the Achilles. He's working very hard on the things he can work on which is upper body strength and his weight is fantastic.
"So once he comes back into training it will just be the football and the quickness of that he'll have to pick up on and that takes time.
"Theses guys would have been training for four or five weeks and it's not easy to pick up on that having not done that training.
"He probably won't be fit for the start of the season but he'll be pretty near."