The Hammers' record signing has been struggling with a heel injury for the past few months and is under rehabilitation in order to have the problem fixed by the end of preseason according to McDonald.
"He is well under way in his rehab, he is working hard on the training ground and we just have to be a little bit patient," he told Sky Sports News.
"The injury that he has is quite extensive and it takes time for the body to heal itself. He is doing everything he can to push on.
"He has been working really hard and there is still plenty of time for him to get himself fit for the start of the season."