"Donncha has not recovered from his unfortunate knee injury and is out," said Director of Coaching Rob Penney. "There are four or five others who are marginal so we are patchworking together a couple of squads.
"Peter is still suffering from a sore foot - he would probably be the main one - and Keith is 60/40 with his shoulder injury.
"He is taking a little bit of contact and we will see what happens during the week and a call will be made before we travel."
Howlett had long been a doubt with shoulder damage and now the absence of the experienced O'Callaghan is a setback Munster could well have done without against the form team in Europe, the only club left with a 100 per cent tournament record this season.
"Having four All Blacks as back-up suggests they have got a bit of depth," said Penney. "Have they got any weaknesses?
"Every team has some vulnerable areas - though not all of the time or in the same areas - so it will depend on the squad they pick.
"What they have got is the ability that as soon as there is a semblance of an opportunity they just ramp up the intensity massively and leave you behind.
"It is what they do from unstructured play with so much firepower they have in Nalaga, Sivivatu, Fofana and Rougerie that they turn the heat up on you really quickly and you are left floundering.
"It is going to be massively challenging but hopefully we have been able to identify some areas we can have a crack at and see if we can create some breaches."
Clermont, who have won their last 59 home matches, will enjoy "home country" advantage at Montpellier with Penney applauding their ability to ride the pressure of maintaining that fantastic home run.
"Clermont have coped with the pressure exerted on them really well with some good performances on the road as well," he added. "We are basically playing a national side."