Mastny, now with Vysoina Jihlava, had decent experiences with the Under-23 side, but is now happier in the Czech Republic.
"I was a bit down when I was in England and not playing," the 22-year-old admitted, speaking to the Manchester Evening News.
"But I have got a second wind since I came home."
"It is always nice to have someone like him to support and encourage you," added Mastny on his United experiences.
"They (the first team goalkeepers) tried to help me and I tried to learn."
"It was hard because it is a new country, new team, new environment and new people," he continued.
"I am not the type of person who the room is for them. I am not the guy who just speaks to everyone, especially when I was 16 and didn't speak the language.
"I was hoping to play games and show what I can do on the pitch, which unfortunately I couldn't.
I was sad after that season, I didn't achieve what I wanted to achieve. The next season was better, I played almost everything but then Covid came."