There may not be a battle at all. McClellan was more than a little cautious, and Lee's poorly supplied troops cannot stay in the field indefinitely. In this case, the Emancipation Proclamation is delayed until a large enough Union victory. The Battle of Iuka probably isn't big enough, perhaps the Battle of Perryville, when Buell's stops Bragg's attempted invasion. That might see Buell transferred east, where I expect little of him.
Still, McClellan will probably be pressed into a battle which he will manage poorly, just like so many others. Still, if the Army of the Potomac could surive the Seven Days Battles where they were deep in enemy territory and basically leaderless due to McClellan's panic, they can survive this battle.
Afterwards, like OTL's Antietam, Lee's Army of Northern Virginia will have to withdraw, so it will be proclaimed as a Union victory and the Emancipation Proclamation will be issued. Lincoln will also pull McClellan for his inability. Where it might have more longreaching effects is if someone important is killed or wounded that wasn't at Antietam and that could go either way. Loss of Hooker or Meade would hurt the Union longterm, loss of Burnside would probably help.