However, Xavi's side remain top of Group H with 10 points and will reach the knockout stages if they avoid defeat to Porto in their next match on 28 November.
Reigning European champions Manchester City have already qualified and will also advance with a 100% record if they win at Red Star Belgrade on Wednesday in Group G.
Chelsea fell behind early on when a defensive error led to Mateo Joseph scoring his first senior goal as Leeds went ahead after only nine minutes. But the visitors, second in the second tier, were only leaders for six minutes.
Putting this claim to the test, someone decided to do some dropping of every iPhone model ever made, and determine if there is a shift in durability as the screens get larger and larger, as would be expected.
Jurgen Klopp's side have only been beaten once so far this season and went two points clear of Arsenal at the summit despite some wastefulness in front of goal.
It looked as if Marcus Rashford had rescued a point for the visitors when he capitalised on a blunder from Forest keeper Matt Turner 12 minutes from time.
Zidane can only hope the news on his injury will be better than that on Marco Asensio, who could miss most if not all of the La Liga season after rupturing a knee ligament against Arsenal on Tuesday.
"If we want to be a contender we have to show every day and when we are playing those matches, we have to show everybody our wishes. I think we did it," Emery told a news conference.