Every training ground in the country has Sky Sports News on so when Jose Mourinho and his centre-backs looked up and saw that there would be no Harry Kane on Saturday they would all have been dancing around.
I was at the game at Wembley on Sunday and whenever you see a player reach for the back of their leg like that you do feel that it's a major problem.
Less than a week ago, all the talk was about how fantastic Tottenham were as they played Liverpool off the park. All of a sudden, you go out of the cup at home to West Ham and are going into a game against Manchester United without Harry Kane.
As much as they will all say it doesn't matter, this is Harry Kane we are talking about. When he is not fit then Spurs are a different team. It is still a dangerous team and it is still an incredibly strong team, but it is just not as strong as it would have been if Harry Kane were playing. I fancied Spurs but it makes it more of a 50-50 game now.
Mauricio Pochettino knows the value of this player. Now he is going to earn his money.
Tactically, he has options. He could go with Fernando Llorente but I have this sneaky feeling that he will go for mobility against Manchester United and pick Heung-Min Son.
People might say that would be a real kick in the teeth for Llorente but it doesn't matter. If it were Crystal Palace at home on Saturday, you could do that. But at United I think he will want pace.
Son did a really good job last year when Kane hurt his ankle. He can lead that line and he is in really good form. He was fantastic at Wembley last week and he'll be confident.
If Pochettino can get Mousa Dembele back into the team after he played 20 minutes in midweek then that gives him the chance to play Dele Alli further forward in support too.
Pep Guardiola spoke about Tottenham being just 'the Harry Kane team' and as a player in that dressing room that is going to hurt. So I think it is up to these Spurs players now to show that they are more than just Harry Kane.
As for Manchester United, it is a big opportunity for them to lay down a marker and put last week's disappointing result at Huddersfield to bed. It is a game that could change their season because they have had a bit of a sticky spell and had to face a bit of criticism.
It is a huge game for Romelu Lukaku too, up against his countrymen Toby Alderweireld and Jan Vertonghen. There is still that question mark about him when it comes to making an impact in the big games.
There are not many bigger than this one for them because they are playing one of their rivals for the title. This is their chance to show that they are the better team.
Jose Mourinho would not have been too disappointed by a draw if Harry Kane were playing. Now it becomes a three-point game for Manchester United. They will expect to win it.