The circumstances of the injury are by now well documented - the 29-year-old was running out at Murrayfield for the warm-up prior to last month's friendly against Scotland, his right leg suddenly went from under him and the left also gave way.
The concern was immediate - would he be able to play at the World Cup?
"Honestly speaking, it was difficult in Scotland. It was difficult lying in the MRI machine. It was hard to stay positive because I was convinced at that stage that I'd done a little more damage than I had," Horgan explained.
"I was extremely worried as it was only a couple of weeks before the start of the World Cup. No matter what sort of an injury you have, it's not a great time to get it.
"I felt a little pop and I knew immediately that it was medial ligament related. Those things can take a long time. Thankfully when we got the scan, it was a grade two tear and that gave me the opportunity to get back."
It has been a tough road back from injury for the Boyne clubman, who, after a gradual rehabilitation, has made rapid progress over the past two weeks.
"My leg was in a brace for a couple of weeks so I didn't have full movement but I was able to do some weights pretty quickly and after I was given the go-ahead to take off the brace, it was a steady increase in recreating the movements that you would do on the pitch.
"The last week and a half I was back on the pitch, running with the guys and doing full fitness sessions. I was back taking part in full contact sessions in the last few days. I am itching to get back on the pitch now. I can't wait to get involved."
The 6ft 4in winger could help to ingnite Ireland's World Cup challenge this weekend after their flat start against Namibia, and he is adamant that he will do his utmost to make his mark on the tournament - it is far from a case of rushing back from injury to pick up another World Cup cap.
"You come back and you play in the World Cup, but it's not just about getting back and getting out on the field. I don't think that's acceptable or that's right. I want to play to the best of my ability. I'm probably not going to get to another World Cup, so I'm not going out there just to make up the numbers.
"I wouldn't come back from injury just for the sake of getting back. I'd be taking advantage of my team-mates if that were the case."
So having watched his team-mates from the stands last Sunday and taken in the television coverage of Georgia's opener against Argentina, how does Horgan feel Ireland should approach Saturday's Pool D meeting with Malkhaz Tcheishvili's side?
"It is about playing pretty simple rugby and everyone doing their job. I think when you are playing against what is perceived to be a weaker nation - although Georgia will be stronger than Namibia - everyone wants to get their hands on the ball and play a really elaborate type of rugby and that isn't always the smart thing to do," he said.
"I think when everyone does their job correctly, we play our best rugby."