Robins had expected Clarke to be out for six to eight weeks, but the central defender could be sidelined for up to three months after damaging medial ligaments in the weekend defeat to Birmingham.
"The news is disappointing for Peter Clarke and the club because he has been a virtual ever-present and a very strong influence on the team since I took charge," Robins told the club's official website.
"It is a big blow for everyone concerned.
"Peter has the character to come back quicker than expected and it wouldn't surprise me if he did.
"Although we can't put an exact timescale on his return, it will be somewhere in the region of 10-12 weeks."
Clarke, 31, has played in all Town's fixtures this season, bar the Capital One Cup defeat at Hull in September.