Fabrice Muamba is critically ill in intensive care at the heart attack centre of the London Chest Hospital.
The 23-year-old collapsed in the 41st minute of Bolton's FA Cup quarter-final against Tottenham, with the match abandoned after the player received treatment on the pitch from paramedics.
Bolton manager Owen Coyle accompanied Muamba to the hospital and emerged on Sunday morning to give an update on the player's progress.
He told Sky Sports News HD: "Fabrice is critically ill. The next 24 hours are going to be absolutely crucial."
He continued: "We've obviously been inundated with people wishing him well and we hope that if everybody can pray strongly tonight that Fabrice is able to recover.
"It's very serious. There's no getting away from that. He's critically ill and God willing he makes it through."
The club released a statement on Saturday which read: "Bolton Wanderers can confirm that Fabrice Muamba has been admitted to the heart attack centre at London Chest hospital where he is currently in a critically ill condition in intensive care.
"The club has requested the media to respect his family's privacy at this time."
A further update on the condition of Muamba is expected to be released at some point on Sunday morning.
The former England Under-21 star underwent intensive treatment amid worrying scenes at the north London ground.
He was wearing an oxygen mask and was then taken to the London Chest Hospital in Bethnal Green in east London.
Within a second of seeing Muamba fall to the ground, Bolton manager Owen Coyle shouted "he's just collapsed" before running on to the pitch to be alongside his player amid a sombre atmosphere at Tottenham's ground.
Muamba was carried off on a stretcher and referee Howard Webb abandoned the match moments later.
Premier League chief executive Richard Scudamore sent his organisation's best wishes to Muamba and his family, as well as paying tribute to the medical staff at the ground.
He said: "The thoughts of the Premier League, its clubs and players are with Fabrice Muamba, his family and Bolton Wanderers.
"We would like to praise the players, match officials, coaching staff and medical teams of both clubs at White Hart Lane for their swift actions in attending Fabrice.
"The League would also like to commend the compassion shown by the fans of Bolton Wanderers and Tottenham Hotspur."
Worried fans and players took to Twitter to give their reaction to Muamba's collapse.
Middlesbrough defender Justin Hoyte, who came through the Arsenal youth team with Muamba, posted: "I seriously hope my best friend in football is OK. Stay strong bro please please stay strong. God is with you remember that."
Muamba's Bolton team-mate Stuart Holden wrote: "Praying for you Fab. Hope he's OK. Thoughts with him and his family."