The 32-year-old joined the Premier League club as a free agent on transfer deadline day, having left Fulham at the end of last season, but he was not included for his debut in the 0-0 draw against Southampton on Sunday.
The Croatia international was brought in to provide depth in attack for West Ham, with targetman Andy Carroll struggling with a foot injury.
And McDonald is hopeful that Petric may be able to feature next weekend, with West Ham seeking to address a goal drought that has seen them fail to score in their last three Premier League fixtures.
"He's been training by himself in the summer, he has not done any football so we are just trying to get him onto the grass and get him involved in football," McDonald said.
"To be fair to him, he has looked after himself fitness-wise but football-wise he needs a little bit more time and we've got a good week leading up now into Everton and hopefully he might be able to come into the squad if he gets through the week."
McDonald also confirmed that the club are still waiting for news on Carroll, with the 24-year-old having suffered a setback in his latest recovery from injury.
"We are still waiting for the prognosis to come back and see where he is at and how long he is going to be so we are just waiting for the specialist to get back to us on that," he added.
