"My impression was, and is, that General Lee, standing under his trenches, would have been stronger against Hooker than he was in December against Burnside, and that he would have grown stronger every hour of delay, while Hooker would have grown weaker in morale and in confidence of his plan and the confidence of his troops. He had interior lines for defense, while his adversary was divided by two crossings of the river, which made Lee's sixty thousand for defense about equal to the one hundred and thirteen thousand under General Hooker. By the time that the divisions of Pickett and Hood could have joined General Lee, General Hooker would have found that he must march to attack or make a retreat without battle. It seems probable that under the original plan the battle would have given fruits worthy of a general engagement. The Confederates would then have had opportunity, and have been in condition to so follow Hooker as to have compelled his returement to Washington, and that advantage might have drawn Grant from Vicksburg; whereas General Lee was actually so crippled by his victory that he was a full month restoring his army to condition to take the field. In defensive warfare he was perfect. When the hunt was up, his combativeness was overruling. . . .
". . . .The battle as pitched and as an independent affair was brilliant, and if the war was for glory could be called successful, but, besides putting the cause upon the hazard of a die, it was crippling in resources and of future progress, while the wait of a few days would have given time for concentrations and opportunities against Hooker more effective than we experienced with Burnside at Fredericksburg. This was one of the occasions where success was not a just criterion."
Source: From Manassas to Appomattox
Commentary
Many historians have called Chancellorsville Lee's greatest victory. From a purely tactical perspective this may be true. But the effect of the battle on the Army of Northern Virginia was devastating even in the absence of the death of Stonewall Jackson.
From his memoirs, Longstreet's comments on the battle of Chancellorsville: