David Moyes saw his injury worries mount up as West Ham were held to a 1-1 FA Cup draw by Bristol City.
The Hammers looked on course for a comfortable afternoon when Jarrod Bowen fired them into an early lead.
Lucas Paqueta set up Bowen's goal after just four minutes, but limped off shortly after with a recurrence of a knee injury.
Defender Konstantinos Mavropanos was also forced off with a shoulder problem before half-time. But most worryingly of all, top-scorer Bowen had to be helped from the pitch after the final whistle having gone down injured in stoppage time.
A second-half equaliser from Tommy Conway secured a replay for the Robins.
The injuries may have brought into question Moyes' decision to play his strongest team against the Championship side.
But the Hammers boss insisted: "I had no intention of doing anything else than making sure we put out as strong a team as we possibly could.
"Injuries are part of football, and unfortunately we picked up a couple today. Losing Lucas was a big turning point in the match."
West Ham's squad would have been given a whole a week off had they won this third-round tie, but they will now be dragged back in on Friday as Moyes, who reached Wembley twice as a player with City, prepares for a return to Ashton Gate.
Almost 9,000 members of City's cider army - among an impressive 62,500 sell-out - made the trip to the capital, but the raucous bunch who filled the Sir Trevor Brooking stand were silenced after just four minutes.
Paqueta dropped deep to collect the ball and lifted a delicious pass over the top to Bowen.
The England hopeful still had work to do, controlling the ball before knocking it past the dive of City 'keeper Max O'Leary and beating covering defender Cameron Pring on the goal-line.
Sadly for West Ham it was Paqueta's last involvement in the match. The Brazil playmaker was only just back from a knee injury and seemed to suffer a recurrence.
Teenage striker Divin Mubama was sent on as a replacement for a rare chance to impress.
West Ham almost doubled their lead when Bowen got round the back of the City defence and pulled the ball back, but O'Leary made a superb reaction save to claw out Pablo Fornals' shot.
O'Leary made another fine stop to prevent an own-goal from Pring, who inadvertently turned Bowen's cross-shot towards his own net, and then tipped a James Ward-Prowse volley wide.
Moyes was forced into a second substitution after just 38 minutes when Konstantinos Mavropanos was hurt after an aerial challenge with Conway, with veteran defender Angelo Ogbonna sent on.
City, 11th in the Championship and on a run of one defeat in five matches under former Hammers Under-23 coach Liam Manning, threatened sporadically in the first half.
But Sam Bell shot straight at Lukasz Fabianski and Rob Dickie's header was also too close to the Polish keeper.
City should have drawn level early in the second half when a low cross from captain Jason Knight eluded everyone in the box and fell to Pring, who lashed his shot wide at the far post.
But on the hour mark the away fans behind the goal were delirious when Joe Williams pinged the ball forward.
Ogbonna missed it and Conway raced forward before burying his shot across Fabianski and into the net.
West Ham poured forward in a bid to avoid a replay - and preserve their week off - but Tomas Soucek headed over and substitute Danny Ings hit the sidenetting.
A replay is the last thing Moyes needs as the injuries begin to bite, but he claimed: "I'm looking forward to going back to Bristol, I've not been there for a long time, I'm really looking forward to going back to Ashton Gate.
"The amount of games we played this season it would be better if we didn't have it, but if I was Bristol City I'd be thrilled to have West Ham at Ashton Gate."
What's next?
West Ham's next game is on Sunday January 21 when they travel to Sheffield United in the Premier League; kick-off 2pm. The Hammers then host Bournemouth on Thursday February 1; kick-off 7.30pm.
Bristol City's next outing is at Preston in the Championship on Saturday; kick-off 3pm. The Robins then host Watford on Saturday January 20; kick-off 3pm.