The 30-year-old winger, who has once played for Tottenham and Leeds and has won 21 England caps, was taken to the hospital on Sunday to be assessed, after officers were called to Eccles Old Road in Salford.
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.
“Officers attended and a 30-year-old man was detained under Section 136 of the Mental Health Act and was taken to hospital for assessment.”
Lennon’s representative, Base Soccer Agency, tweeted: “Everyone at Base Soccer sends their support to @AaronLennon12 – get well soon and stay strong.”
Lennon, who joined Everton in a permanent £4.5m in September 2015, has not featured for the Toffees since February 21.