Chelsea suffered another injury setback on Saturday when Kai Havertz limped off during the match against West Ham.
Havertz once again led the Premier League leaders' attack in the London derby, with manager Thomas Tuchel naming Romelu Lukaku and Timo Werner on the bench.
The German international squandered a couple of chances during an eventful first half in Stratford, most notably firing into the side netting after Mason Mount - who was almost certainly offside - had reached the byline and squared the ball across.
At the London Stadium, Chelsea took the lead through Thiago Silva's header from a corner, only for Manuel Lanzini to equalize from the spot after Edouard Mendy brought down Jarrod Bowen following a woeful backpass from Jorginho.
Chelsea did reclaim a narrow lead before the interval thanks to Mount's sublime volley, and Havertz looked as if he might be in for a stoppage-time goal to make it 3-1.
The former Bayer Leverkusen forward, however, was denied by a superb sliding challenge from ex-Chelsea stalwart Kurt Zouma.
While the tackle was entirely justified, it left both players on the floor and in need of medical attention.
While Zouma was able to shake off his issue and return for the second half, Havertz appeared to be in significant discomfort and had to be helped off the pitch to make way for Romelu Lukaku.
Zouma too, however, left the field of play in the 71st minute with what appeared to be a hamstring injury.
The game ended in a 3-2 win for the Hammers who will now finish the weekend in the top four, while Chelsea may lose their lead at the top.