ABUJA, Nigeria  The circumstances of Acting President Goodluck Jonathan’s accession to power are so odd that even he looks bewildered as he takes a self-effacing bow in this boiling, fractious nation.

He has not been elected. He has not exactly been appointed. He did not seize power in a coup, unlike many of his predecessors. And as a mild-mannered academic in a black fedora, he seems an unlikely fit in Nigeria’s tough-guy environment.

But his boss, President Umaru Yar’Adua, is very sick, incommunicado for nearly three months in a Saudi Arabian hospital. With no word on his return, the National Assembly last week promoted Mr. Jonathan from vice president to acting president, calling the move essential to “peace, order and good government.”

So it was that Mr. Jonathan, appearing before fellow West African leaders at a summit meeting here this week, bent his head, mumbled greetings and apologetically called himself a mere “stand-in for President Yar’Adua, who is unavailable.” So unavailable, in fact, that a National Assembly delegation inquiring about the president’s health last week in Saudi Arabia was unceremoniously turned away.