Denis Zakaria scored the winner on his Chelsea debut as the Blues came from behind to beat Dinamo Zagreb 2-1 on Tuesday.
But the night ended on a sour note for Chelsea and England when left-back Ben Chilwell pulled up with what looked like a hamstring injury that could end his hopes of going to the World Cup.
The Croatians had more to play for at Stamford Bridge with Chelsea already assured of top spot in Champions League Group E.
Victory would have secured Dinamo a place in the Europa League knockouts and they made a flying start when Bruno Petkovic headed in after seven minutes.
Defeat in Zagreb when the sides last met in September was the final straw for the Chelsea board before they sacked Thomas Tuchel.
New boss Graham Potter was beaten for the first time in 10 games in a 4-1 thrashing on his return to Brighton at the weekend.
Potter named a stronger than expected line-up in response and two of his star names combined for the equaliser as Pierre-Emerick Aubameyang's backheel teed up Raheem Sterling for his first goal in nine games.
Zakaria has had to be patient for an opportunity since joining from Juventus on loan on transfer deadline day.
There have even been reports of the Swiss midfielder's loan being cut short in January, but he made his mark by side-footing into the bottom corner from Mason Mount's cross.
Aubameyang hit the bar in the closest either side came to a goal after the break.
But the biggest blow of the night for the English giants came in stoppage time when Chilwell fell to the floor clutching his hamstring before needing assistance to hobble off the field.