(CNN) -- Fabrice Muamba's fellow footballers rallied around the Bolton player after he was admitted to hospital following a possible heart attack during a match on Saturday.
The midfielder was in critical condition in intensive care, the English Premier League club confirmed.
He collapsed for no apparent reason four minutes before halftime in the FA Cup quarterfinal at Tottenham in London, and the match was abandoned.
"Come on Fabrice Muamba, praying for you," Manchester United star Rio Ferdinand wrote on Twitter.
Ferdinand's clubmate and fellow England international Wayne Rooney added: "Hope fabrice muamba is ok. Praying for him and his family. Still in shock."
Arsenal's England midfielder Jack Wilshere played with the 23-year-old while on loan at Bolton during the 2009-10 season.
"Hope Muamba is okay. Thoughts with him," the 20-year-old tweeted.
Muamba began his career at Arsenal, where he came through the youth ranks with defender Justin Hoyte, who now plays for second division club Middlesbrough.
"I seriously hope my best friend in football is OK. Stay strong bro please please stay strong. God is with you remember that," tweeted Hoyte, who like Muamba has represented England at under-21 level.
Muamba was born in the Democratic Republic of Congo, then known as Zaire, and came to England in 1999 at the age of 11 after being granted political asylum.
He told the UK's Daily Mail newspaper in 2008 that his family had fled due to his father's previous position working for the overthrown regime of ex-president Mobutu Sese Seko,
"It was very, very tough," Muamba said. "I saw the war. I saw people die. I grew up with it. It was scary.
"I didn't live far from the gunshots and the sound of them going off. It was difficult to get used to, especially hearing guns at night. It did have an effect on me.
"It stopped us going out to play football because we were scared we would get killed. One or two of my friends were hurt, one or two of them have since died."
Muamba is said to be deeply religious, the UK Press Association reported.
"Fabrice is an extremely strong believer in God and says that he is the reason for everything he has done and accomplished," according to his profile in a match-day booklet.