The rugby player, regarded as one of England's best, made his return to the sport over the weekend after suffering three arm breaks in just over a year.
Despite scoring a try he was unable to prevent his club side, Saracens, losing to Sale.
Vunipola says on previous returns from injury his confidence was counter-productive and often saw him injured straight away.
Speaking to the Telegraph, he said this time it was different: "I was a bit tentative and anxious - I readily admit that now.
"Before I was a bit naive that I could fall straight into it, whereas now I am a bit more prepared. And scared as well. Not of being injured, but the process of coming back is the hardest thing. Hopefully this weekend I can be better, then next weekend, then see when you go from there.
"That is probably my faith. I think God wanted me to be afraid. Before I was so confident I would be all right. I guess a bit too confident to the point that 'I'm invincible'. That's why I got injured again. So I'm trying to find my feet."
Vunipola's return to rugby is timely for the national team as they head into the Six Nations tournament in February.
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