And Richmond young gun Sam Lalor will miss up to six weeks with a hamstring injury, while Fremantle's Cooper Simpson is set for surgery on his left shoulder.
McKay won't play in the Bombers' match against Brisbane at the Gabba on Thursday night due to a foot injury suffered when trodden on in the Bombers' Dreamtime win over the Tigers last Friday.
Essendon coach Brad Scott says the club are yet to determine a return timeline.
"He won't play this week. The short turnaround is going to make that too difficult," Scott said on Monday.
"We're still assessing. He had a scan this morning, so we'll get those results."
The timing couldn't be much worse for the Bombers, who also lost forward Kyle Langford (quad) and key defender Zach Reid (hamstring) against Richmond.Â
Scott does not expect to see the duo before their round-16 bye.
Langford, a two-time leading goal-kicker at Essendon, has already missed six games this season due to a knee injury.
The trio join Jordan Ridley (hamstring) and Harrison Jones (ankle) on the injury list, with Nick Bryan (knee), Thomas Edwards (knee), Lewis Hayes (knee) and Sam Draper (achilles) suffering season-ending injuries.
Scott could turn to Darcy Parish after the midfielder played his first game of the season in the VFL following back and calf setbacks.
But Saad El-Hawli has also been ruled out, suffering a hamstring strain in the reserves.
With the mid-season draft slated for Wednesday, Scott said there was a chance a rookie could be handed a debut on Thursday.
"It's not beyond the realms of possibility - we might have to look for Queenslanders; a bit logistically easier," Scott joked about the Gabba match.
Richmond midfielder Lalor has re-injured the hamstring he hurt before being taken with the No.1 pick at last year's national draft.
The Tigers on Monday confirmed Lalor was expected to miss four to six weeks.
"Sammy Lalor had a re-aggravation of his hamstring injury that he sustained prior to coming to the club," Richmond high performance boss Ben Serpell said.
"Unfortunately for him, he is going to miss a bit of game-time.
"Thankfully it's only going to be around about three games though."
Teammate Thomson Dow (dislocated kneecap) will be sidelined for up to eight weeks, while captain Toby Nankervis will return against GWS on Saturday after being managed last week.
Key forward Tom Lynch still has to get through the later stages of concussion protocols before being cleared to return and Jayden Short (calf) is another week or two away.
Fremantle defender Simpson will go under the knife after injuring his left shoulder in a heavy hip-and-shoulder from Port Adelaide's Logan Evans.
The 20-year-old is expected to return to action late this season.
Simpson's injury comes as Dockers captain Alex Pearce faces a three-match ban for a heavy collision with Darcy Byrne-Jones.
"This is a relatively rare injury and the best intervention is to head in for surgery," Dockers football boss Joe Brierty said in a statement."We will know more on timeframes post the procedure, but at this stage we expect Coop to be back playing in around 10 weeks."
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