It was desperately cruel luck for the 23-year-old, who was thought to be one of the front runners for a place in the Lions Test side to face South Africa on Saturday week.
Ferris scored two tries in his first two appearances on tour against the Golden Lions and Cheetahs, winning the man-of-the-match award in the latter game.
But remaining positive and true to his character, he is already turning his attention to getting back playing with Ulster and Ireland and building on his Grand Slam and Lions achievements.
"It's probably a four-week injury and I'd be really fighting to get fit for maybe the last Test match," Ferris said, reflecting on missing out on the Test series.
"It's just one of these things. It was just training. I was doing a bit of tackling and I twisted my knee.
"A couple of guys fell on me and I heard a small pop on the inside of my knee. It was my MCL and a grade two tear.
"As my mother's always told me, there's a silver lining to every cloud. You just kick on and look forward to a big season next year.
"I'll go back to Ulster now, get myself fitter, stronger and faster and come back a better player, I'm sure, next season."
Ulster's 2009 Lion was a much-liked member of the touring party in South Africa and he will certainly be missed both on and off the pitch.
Although the end has been an abrupt one, Ferris will always look back on the summer of 2009 with fond memories and Australia 2013 should offer him the chance to don the famous red jersey once again.
"It's been absolutely fantastic and I've enjoyed every second that I've been here.
"I've met some great friends and it's been great to be a part of such a lovely bunch of guys. Hopefully I'll get another opportunity to do that again in a few years' time."
Commenting on Ferris' injury blow in his Irish Times tour blog, his Grand Slam-winning colleague Tommy Bowe said: "Typical Stephen, he sat in the team room and put a brave face on, trying his best to be positive, but I can only imagine what must be going on in his head.
"He has been outstanding on this trip so far and in my opinion was a shoo-in for a Test place.
"I'm very friendly with Stephen and am absolutely devastated for him, he has become one of the most popular men in the squad and every single player here feels terrible seeing him walk around on crutches."