Everton say winger Aaron Lennon was detained under the Mental Health Act by police over concerns for his welfare.
The 30-year-old was taken to hospital "for assessment" after police were called to Salford on Sunday.
Lennon is now "receiving care and treatment for a stress-related illness", his club has said.
The England international, who joined Everton from Tottenham in 2015, has not played for the first team since February.
Greater Manchester Police said: "Police were called at around 4.35pm to reports of a concern for the welfare of a man on Eccles Old Road.