The Red Devils swept to the Premier League crown in 2012/13, offering the perfect send-off to outgoing boss Sir Alex Ferguson.
A changing of the guard at Old Trafford has not run smoothly, though, with new manager David Moyes enduring a baptism of fire.
There have been signs in recent outings that the tide is turning, but a slow start has left the defending champions eight points off the pace.
Toure feels United will find it difficult to bridge that gap, with Liverpool considered to be a greater threat to the usual suspects in the battle for top flight supremacy.
He said in The Sun: "United will find it hard to defend the title since they have a new manager and are really struggling.
"Liverpool are a big threat at the moment because they have a strong squad and their strikers - Luis Suarez and Daniel Sturridge - are scoring goals at will."
City ended their latest outing empty-handed, leaving them just two points better off than United and with goalkeeper Joe Hart coming in for renewed criticism.
The England man endured an afternoon to forget during a 2-1 defeat at Chelsea on Sunday, but Toure is convinced he remains the right man to provide City's last line of defence.
He added: "Joe is a top keeper who gives his all to the team."