YK was President, Mujeeb was standing to be PM. As it was YK had stated on multiple occassions that Mujeeb would be PM before March 17 1971 when the had their last meeting.
It is usually forgotten that in the weeks leading up to the events of March 1971 E Pak had become ungovernable, with Mujeeb's goons controlling the streets and the police and local government had all but lost control. Hundreds of non Bengalis were killed, raped or robbed. YK had told Mujeeb that if he could call of the AL mobs he could be PM but Mujeeb refused to do so.
Furthermost, Mujeeb was in liason with Indian government officials which infuriated YK who knew about the links as the ISI had heavily penetrated the Indian establishment in the late 1960's. So YK did not believe in Mujeeb's sincerity and again he was right to doubt it.
He was also right to have launched the operation, BUT and this was his mistake, he should have kept it limited to restoring order, which in OTL was achieved by May 1971. That was an opportune moment, if he had then called for a government to be formed, I think it could have succeeded,