The England attacker is currently on-loan at Barcelona with the deal carrying a €30m permanent option. Recent speculation suggests United management have no plans to bring back Rashford should Barca pass on buying him outright.
For Thornley, the club's stand is no surprise. He told The Busby Way Podcast: "I'll say it, I don't mind, I know Marcus, and if he's not happy with what I say, I don't care.
"He became unhappy. He became a different person. I watched this player that used to be a happy, lovely kid just moping around, especially in the club.
"Outside the club, I used to see him every now and then, and he was fine outside the club, but in the club, he had this demeanour about him that just wasn't great. Then it transferred onto the pitch, and then you see him walking around, and you just wanted energy levels out of him, which is what Michael Carrick is doing now.
"He's getting energy levels out of the players, and you wanted that out of Marcus, but you just weren't getting it, and it was tossed off basically. For me, he's half tossed off his Manchester United career rather than it has been taken away from him."
Thornley was team masseur at United between 2010 and 2019.