Mainoo has become a prominent player for both United and England since his first team breakthrough last November.
The 19-year-old scored in the FA Cup final win over Manchester City in May and also started in England's Euro 2024 final defeat by Spain.
The midfielder had a day to forget against Liverpool and was at fault for the third goal as he was pickpocketed by Alexis Max Allister, with Mohamed Salah eventually scoring in a 3-0 win for the Merseyside club.
And Souness believes that while Mainoo might become 'a player' in the future, he's currently 'not the go-to man'.
Graeme Souness has claimed that Kobbie Mainoo is not good enough to be a regular starter
'I have to say this, I'm not being unkind, Mainoo might be a player, but it's so Man United,' Souness said speaking to talkSPORT.
'This is not to be critical of Man United, it's never been any different.
'But when someone comes on the scene at Man United, in my days they were the "new George Best", or the 'new Bobby Charlton', the "new Denis Law".
'It is FC Hollywood, they call Bayern Munich in Germany "FC Hollywood", that's Man United here.
'If anyone shows anything, they're put into superstardom category, way before they've even become a good player.
'And Mainoo right now should be a player that's flitting in and out of a good team - not coming in and being asked to stand up and play against the very best teams.
'He might be a player one day, but he's certainly not the go-to man at United like they're making him out to be in midfield.'
Mainoo has started all three of United's league games so far this season, partmetomg Casemiro in all of them.
He is part of the England squad to play Republic of Ireland and Finland in the UEFA Nations League