When Bolton midfielder Fabrice Muamba collapsed on the pitch at Spurs on Saturday evening, it was immediately clear something was seriously wrong.
Muamba, just 23, had suffered a cardiac arrest and, following six minutes of emergency treatment by paramedics on the pitch, he was rushed to the London Chest Hospital, where he remains in a critical condition in intensive care.
Speaking on BBC's Match of the Day, Sir Alex Ferguson said the incident "puts everything into perspective" and praised the medical staff at White Hart Lane, while many of his Manchester United players used Twitter to express messages of support.
Wayne Rooney: "Hope Fabrice is ok. Praying for him and his family. Still in shock."
Rio Ferdinand: "Come on Fabrice Muamba, praying for you."
Tom Cleverley: "Not watching the game but I hope Fabrice Muamba is ok!"
Michael Owen: "My thoughts are with Fabrice Muamba."
Thousands of United supporters have already responded to a message posted on the club's official Facebook Page on Sunday: "Best wishes to Fabrice Muamba and his family from everyone at Manchester United."
Show your support: You, too, can post messages for Muamba on our Talking Reds forum at manutd.com/messageboard. We'll be sure to pass them on to Bolton.