His expertise was never replaced and his absence has contributed to the group's transition from a fear-inspiring attack force to a hate filled voice on the Internet, urging others to wage terror against the West.
"In terms of competence for managing, planning and executing terrorist attacks, KSM was the best in al Qaida," said Rohan Gunaratna, a terrorism expert and author of a book on al Qaida. "That's why Osama bin Laden and other important al Qaida leaders entrusted him with so many operations."
But even if his self proclaimed status is somewhat exaggerated, he "gave the Americans lots of information about what kind of ideas al Qaida had, and how they put their plots together," said Magnus Ranstorp, a terrorism expert at the Swedish National Defense College.
"Mohammed's information gives us a glimpse into the group's imagination. On an operational level, I don't think they have ever been able to replace his experience," he said.
Experts agree the arrest was a major setback for bin Laden's terror network, and one from which the group has never quite recovered.