According to Dave Meltzer the reason why the UFC has yet to officially announced the addition of Mirko Cro Cop to UFC 99 is that he has not signed his new contract.
Reasoning outside of the UFC/CroCOp negoiations appears to be the reason for pen not hitting paper yet.
The former top ranked fighter is expected to be signed and officially announced early this week.
The Croatian was brought in to help ticket sales for the June event from Germany but with no contract signed the UFC has been unable to officially announce the return of the heavyweight after a dismal first run for the company following the buy out of Pride FC.