The Dutch left-back scored two goals in five Premier League appearances as the 24-year-old deputised for veteran defender Patrice Evra throughout the title-winning campaign.
Buttner joined the 20-time English champions from Vitesse Arnhem last summer, but the defender revealed he would have quit the Red Devils had it not been for his supportive brother and girlfriend.
“I now know about homesickness. I’d think about my family and friends at home many times,” Buttner told the Sun.
“How they were, what they were doing, little things like that.
“But I did not give up and I have my girlfriend Laura and brother Denny to thank for that.
“They were always there for me in England. I could call on them any time. I needed their warmth.
“I’m a fighter. But had I been on my own, I’d have got on a plane back to Holland a long time ago.”
Earlier this month, Vitesse Arnhem coach and former United goalkeeper Raimond van der Gouw praised Buttner’s impact in his first season at the 20-time English champions.
“I don’t get to see United as much as I want to because I’m too busy with the job at my club in Holland,” Van der Gouw told United’s website. “But I think Alex can be pleased.
“He did really well. I think he wants to play a few more minutes of course but it depends on how it goes for next season.
“I read an article from him and he said he had a chat with Patrice Evra and [he’d said] that Alex should be very happy with the minutes he’s played now as it was a massive step from where he was coming from.”