Newcastle manager Eddie Howe was admitted to hospital on Friday.
The 47-year-old will now miss their Premier League clash with Manchester United on Sunday.
The League Cup winner is, however, conscious and in communication with his family.
Howe was kept in hospital overnight and continues to undergo further tests.
“The Magpies’ head coach was admitted to hospital late on Friday evening, having felt unwell for a number of days,” Newcastle said in a statement.
“Medical staff kept Eddie in hospital overnight for further tests, which are ongoing. He is conscious and talking with his family, and is continuing to receive expert medical care.”
Howe’s assistant, Jason Tindall, and coach Graeme Jones will take charge against the Red Devils.
The match is pivotal to Newcastle’s hopes of securing Champions League football next season.