But the 26-year-old Welshman, who recently qualified as a doctor, has somehow made it back to full fitness in time for Saturday's decisive third Test thanks to the hard work, dedication and expertise of the Lions medical team.
"As soon as I was walking off the pitch in the Waratahs game I thought I was going home. I've never had a hamstring strain before, it was alien territory for me," said Roberts, who hadn't run at 100 per cent by the time of last week's second Test.
"It was probably a day or two after when I knew I might have a chance. I had a scan the day after the injury, and then they sat me down and said they would keep me on tour, would stay positive and do everything in their power to get me fit. It's worked in my favour, the Gods have smiled on me.
"I did everything in my power to get back and I'm hugely grateful to the medical team. I have to say a massive thanks to Dr James Robson, and especially Prav (Mathema, Lions physio) who has worked wonders with the rehab. I have to repay that faith. Hopefully I can go out and give them a performance on Saturday because I owe a huge amount to the medical team."
Roberts will partner international team-mate and flatmate Jonathan Davies in the centres on Saturday, with Brian O'Driscoll dropping out of the side to accommodate his return.
The Racing Metro-bound star admits replacing the most experienced Lion in the squad brings with it an added responsibility but it is one he is relishing ahead of the biggest game of his career to date.
"I see it as a responsibility, I don't see it as pressure. I've been picked to do a job and it is important I deliver. I've always thought that I thrive under pressure in big games, it's where I like to play," added Roberts.
"The higher the pressure and the more responsibility is put on me, the more I thrive and the more I enjoy it. The coach picks the team and Warren (head coach Warren Gatland) has picked the team he thinks will go out there and beat Australia on Saturday. That changes week in, week out, with injuries and performances.
"Thousands of people have said it before me and thousands of people will say it after me, Brian is one of the most incredible players I have played with or against in rugby union. To show you what sort of guy he is, he was the first to come up to me at the end of training and ask if I wanted to do any extras. He's one of the greatest players to have played the game. He will be remembered as a Lions legend, I'm sure. Brian is a leader, I've tasted that playing alongside him. He's a force, and it's up to me and Jon now to lead the way in midfield and take that responsibility and thrive under the pressure.
"Over the last three years with Jon, I think we are the most capped centre partnership Welsh rugby has seen. We've got experience, we've played together a lot. Jon has been one of the standout players of this tour. His all-round game is pretty special, he's a great athlete, he can kick a ball, he's a good distributor and he's got an eye for space.
"Hopefully, the dimension I bring in attack and defence can help him unleash his attributes as well. That goes for all the backs: it's not just about my game and what I bring, it's about what I can add to the back-line line dimension.
"There is no bigger stage to play on. You just have to get completely excited about it, not shy away from it, not get nervous. It's just pure excitement, go out and express yourself, keep your concentration and deliver under pressure. It's a Lions deciding Test match, it's all or nothing, and this is the time you have to deliver."