"There is massive belief among the players that we can turn this around over the next two weeks. The series is far from lost," said Roberts, who was one of the Lions' star performers as they came within a single score of overturning a 26-7 deficit at the ABSA Stadium.
"We didn't come out all guns blazing, which we definitely need to do next week.
"We feel we should have won that game and next weekend can't come soon enough for the boys."
The Lions trailed by 19 points with just 12 minutes remaining before tries from Tom Croft (his second of the match) and Ospreys scrum-half Mike Phillips saw Britain and Ireland's elite come so close to producing a remarkable comeback.
"We want to put right the things that went wrong and we need to be more clinical," added Roberts.
"We need to start next week as we finished this game.
"We created some lovely chances, but we didn't make them count. We got over their line three times and failed to get the ball to ground.
"I was happy with some parts of my game and then disappointed with others. I got turned over a few times and those are the fine margins that can make a big difference at this level."
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