Tottenham Hotspur fear that Harry Kane could be out for up to two months, and potentially miss the rest of the season, after suffering an ankle injury in the hammering of Millwall that earned them a place in the FA Cup semi-finals, their first semi-final for five years.
This was Spurs' worst nightmare - not facing Millwall, although there were racist chants aimed at Son Heung-min in the first half and a huge police and stewarding presence with trouble outside the ground afterwards, but losing Kane.
The striker winced in pain as he attempted to carry on and that pain was felt around White Hart Lane in what was the final cup tie to be played in this iconic stadium after 118 years here. What a way to sign off.
Kane is out of Spurs' next Premier League match against Southampton but it could be much, much worse than that with ramifications for the league run-in and his side's fight to finish in the top four and qualify for the Champions League.
Kane left the stadium on crutches, with his right foot in a boot, which is usually a precautionary measure but it was evident in the pained expression of Spurs manager Mauricio Pochettino that he certainly feared the worst.
It was even the same ankle that Kane damaged earlier this season, in September against Sunderland, which ruled him out for seven weeks - a period when Spurs struggled - and he landed in the same way.
"We hope it's not a big issue, but it looks similar. If you watch it on television, it was a similar action," Pochettino said. "Harry is our main striker, one of the best in England, but we can't cry about it now. We have to be positive.
"We have enough players to try and replace him. It won't be an excuse if we don't win or achieve our aims because Harry is not in the team."
Spurs can point to Son's three goals and a first in open play for Vincent Janssen - on his 30th appearance (after 1,134 minutes, to be precise) - but there is no disguising the scale of the loss.
Kane went over under a challenge from Jake Cooper while shooting early on in the tie. The striker received lengthy treatment but eventually peeled off the captain's armband and gingerly hobbled down the tunnel. It was such a pity - not least because he is in such outstanding form.
That Spurs coped so admirably here was to their credit. Yes, it was League One opposition but they were on a 17-match unbeaten run, had dumped three Premier League teams out of the Cup and, as manager Neil Harris later admitted, were motivated to produce an even greater upset. They sensed a little slice of history.
Spurs did not give them a sniff. Son was outstanding, as was Harry Winks, Kieran Trippier and Victor Wanyama while Dele Alli - who could have had two penalties - was on a mission to score and was substituted only once he had done so.
This victory was as emphatic as it comes, in fact, with Pochettino fielding a strong team and delivering a clear message that this is a trophy he wants. And, without one so far in his managerial career, he does need a trophy. As does this Spurs team.
They quickly had a goal. Christian Eriksen, not Janssen, had replaced Kane and after half an hour he latched on to the ball from Alli's chest-down to sweep a first-time shot beyond goalkeeper Tom King.
By now there were unacceptable chants aimed at Son - and a response from Spurs fans - but the focus returned to the pitch with the South Korean then adding to the scoring with a superb left-foot shot.