From Star Wars Technical Commentaries, Novel References Section
Quote
"The Rebels did indeed fight better, but not because of any special abilities or training. They fought better than the Fleet because the Emperor was dead."
[Thrawn] turned to Pellaeon. "You were there, Captain--you must have noticed it. The sudden loss of coordination between crew members and ships; the loss of efficiency and discipline. The loss, in short, of that elusive quality we call fighting spirit."
"There was some confusion, yes," Pellaeon said stiffly. He was starting to see where Thrawn was going with this, and he didn't like it a bit. "But nothing that can't be explained by the normal stresses of battle."
One blue-black eyebrow went up, just slightly. "Really? The loss of the Executor--the sudden, last-minute TIE fighter incompetence that brought about the destruction of the Death Star itself--the loss of six other Star Destroyers in engagements that none of them should have had trouble with? All of that nothing but normal battle stress?"
"The Emperor was not directing the battle," Pellaeon snapped with a fire that startled him. "Not in any way. I was there, Admiral--I know."
"Yes, Captain, you were there," Thrawn said, his voice abruptly hard. "And it's time you gave up your blindfold and faced the truth, no matter how bitter you find it. You had no real fighting spirit of your own anymore--none of you in the Imperial Fleet did. It was the Emperor's will that drove you; the Emperor's mind that provided you with the strength and resolve and efficiency. You were as dependent on that presence as if you were all borg-implanted into a combat computer."