The 22-year-old damaged his posterior cruciate in a collision with Belgium’s Thomas Meunier during Tuesday’s under-21 friendly.
He will miss the next three months, including two European Under-21 Championship qualifiers against Luxembourg and Austria. Should Scotland fail to reach the play-offs, Ridgers will be too old come the start of the next qualifying campaign.
“It could be the end of my Under-21 career because we’ve got two more qualifiers,” he said. “If we win them, we’ve got a chance of making the play-offs. If not, that’s our campaign finished and then I’m too old. It’s disappointing if it ends that way.
“I just need to be positive and hopefully get an opportunity to play for the full national team in the future, which would beat any under-21 cap I’ve had.”
Ridgers revealed how he believed he could play on in the immediate aftermath of the injury.
“I offered to play on,” he told the Hearts website. “The doctor and the physio checked me and both agreed it would be best if I came off. I wasn’t happy at the time coming off, but looking back now I’m glad I did.
“I’ve got to see a specialist on Monday and then we’ll find out the full extent. It’s not just my PCL that I’ve torn – there are other areas around my knee that have to be looked at. It’s disappointing and hopefully it’s not much longer than three or four months out. You never know, if everything heals well, I might be back sooner.
“I’ve got to be as positive as I can but I’m not going to lie, I’m absolutely gutted. I was No. 2 and there was an opportunity to play if there was an injury to Jamie MacDonald or, with his girlfriend due a baby, I might have got another game or two.”