McInnes remains hopeful Zola will be fit, but the injury may affect the manager's plans for Wednesday's match.
He said: "Calvin over-stretched his knee in training (on Saturday). He should be fit for United, but that's why he missed out (against Ross County)."
Should Zola not recover, Vernon and Josh Magennis are the other options for the main striker's berth against United, but it is defence that McInnes will be focusing on in January.
He said: "It's got to be the priority now, with Reading's decision on Michael Hector (he will return to the Royals in January when his six-month loan deal expires), Andy Considine tearing his calf and Clark Robertson still some way away. That's not to say that we won't be looking elsewhere, but we are short and we're just about getting away with it just now.
"There's a few options out there, but we'll assess our options. Clubs are reluctant to give out good players in January, and it's a good player we'll need to improve things."