Munster are hopeful that Craig Casey's injury is not too serious but haven’t ruled out making contact with retired scrum-half Conor Murray.
Casey, who captained Munster to an opening-round URC win over Scarlets last weekend, suffered a thigh injury in training on Tuesday.
The province didn’t provide a timeline for the Ireland international's recovery in today's injury bulletin ahead of Saturday’s clash with Cardiff but head coach McMillan told RTÉ Sport that the best case scenario is a two-week layoff.
"We’re still waiting on the timeline but he picked up an injury at the back end of training," said the New Zealander, who took charge over the summer.
"We don’t know the full extent of it yet but we anticipate that it’s probably going to be, at best, a sort of two-week injury, potentially a little bit longer.
"[It was] just non-contact, a real anomaly, that’s why were just taking our time before we make any announcement around his injury because we are not quite sure what that is at the moment."
Former Munster and Ireland scrum-half Murray retired from international rugby last season and finished up with the province, however, he initially planned to continue his playing career abroad before finally calling time last month when nothing suitable came along.
Munster have scrum-halves Ethan Coughlan, who starts at Thomond Park on Saturday evening, and Paddy Patterson available.
The pair have played 20 and 39 times, respectively for the province, while academy scrum-half Jake O'Riordan (20) is yet to make his debut.
McMillan was asked if the extent of Casey’s injury had led them to consider getting in touch with 36-year-old Murray, who made 340 senior appearances for club, country and the British and Irish Lions.
"Ha, well you never say never," he said.
"No, haven’t had any contact with Conor at the moment.
"Look, we haven’t really been out in the market and gone outside the guys that were here when I first walked in the door.
"Our intention is to give everyone that’s in the building every opportunity to put their best foot forward.
"Obviously we’d have a few concerns if the other two blokes that we’ve got running around took a ding that might put us in a vulnerable position, but we have every faith in them.
"We’ll do our homework behind the scenes if the situation around Craig is more serious than what we initially think to make sure we are protected in that space, but at the moment we are hoping for the best."
Flanker Alex Kendellen (above), meanwhile, has undergone surgery and will be sidelined for six to eight weeks.
"He’s got a high ankle sprain that will require some surgery that will just help accelerate his recovery," said the 51-year-old former Chiefs coach.
"He picked that up at the back end of the game. He defended on the wing for a couple of sets because he was struggling to get going.
"[It’s] one of those unfortunate injuries that sometimes happen in rugby training or games so that puts him out.
"The upside is that we’ve got John Hodnett, who is back in the building, he’s had his surgery to the injury that he got in a pre-season game.
"He’ll be back on the field next week albeit limited around contact but his return to play is looking pretty positive actually in the coming month."
The 2023 URC champions say Tom Farrell (calf), Mikey Milne (calf), John Ryan (elbow), Andrew Smith (chest), Diarmuid Kilgallen (ankle) and Mike Haley (thigh) will return to training shortly, while JJ Hanrahan (concussion) is following return-to-play protocols.
Calvin Nash wasn't considered this week but is nearing a return to full fitness.
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