Mount, who had a year left on his contract last summer, made a big money move from Chelsea to United.
"Mason Mount was totally different at Chelsea he was one of the reasons why Chelsea won the Champions League," said Sagna to Paddy Power.
"He was at home there, and he probably feels homesick by moving to Manchester United it's a different city and club.
"He's used to one way of working, used to the players and staff at Chelsea. It's not always easy when you have to go and start from zero. You can tell that he's struggled to get comfortable in the team he has the qualities to be a very good player.
"But if you don't feel at the right time at the right spot, you'll struggle to perform. Mentally, is he happy to be there? Is he happy to be in Manchester, which is totally different from London? I had a similar change because I went from London to Manchester, and I needed time to adapt.
"He (Mason Mount) had family in London and was settled there, and now he is in Manchester, everything changes. It's about building back that confidence and getting back to the top. He will have more pressure on him next season. It's been difficult for him, but hopefully he's going to have a good pre-season and maybe it's the start of something new."