The burly second row was concussed during his side's heavy defeat to Saracens in the final round of European games, his tenth concussion in the last 22 months.
And with the RaboDirect PRO12 returning this weekend Clarke's absence does not come at the best time for a side desperate to get off the foot of the table.
But despite Clarke's struggles, Lam is hopeful that given the right time away from the game his skipper can return to full action.
"Craig has had his tenth concussion in 22 months, so he is a big concern," he said.
"At this stage he is going to be unavailable indefinitely, until there is a bit more testing and so forth.
"It is obviously a big blow for us but you can just not mess around with that sort of situation.
"The players had a week off after the Saracens game but he was suffering even last week. Obviously we favour investigations but more importantly is just time.
"I had a player before who has had that as well, Benson Stanley, and he was considering retiring and he just had to go through the timing and he was thinking about stepping down.
"But through timing and treatment and rest he got through and he is playing at Clermont now, so we are hopeful."
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