Gates said Zawahiri lacks bin Laden's charisma and operational skills, reports the Associated Press.
Bin Laden "had a peculiar charisma that I think Zawahiri does not have," Gates said.
However, Gates noted, the West must remember that the terrorist organization still exists and is a threat to global security.
“Despite having suffered a huge loss with the killing of bin Laden and a number of others, Al Qaeda seeks to perpetuate itself, seeks to find replacements for those who have been killed and remains committed to the agenda that bin Laden put before them," Gates said in what was expected to be his last press conference.
Al Qaeda appointed Zawahiri to be its new leader Thursday, almost seven weeks after U.S. forces killed bin Laden during a raid on his compound in Abbottabad, Pakistan, on May 2.
Zawahiri, a 59-year-old Egyptian doctor, was bin Laden's deputy. After bin Laden was killed, Zawahri vowed in a video earlier this month to avenge the former leader's death and continue his jihad against the West.
Analysts mostly agree with Gates that Zawahiri does not have bin Laden's ability to inspire young militants and is more known for his "abrasive manner and pedantic speeches," the New York Times reports.
“He’s always been a divisive figure, going back to his years in Egypt,” Brian Fishman from the New America Foundation in Washington told the Times. “He’s just personally disliked by many in Al Qaeda. His personality always gets in the way.”