"It'll be tough because we're missing my brother though we have great players to replace him," Billy told Sky Sports News.
"Also, we're missing Maro so it's probably a good week for me to try and play a better game than I have been in the last two weeks."
Injuries have plagued the 26-year-old in the last few months, with three arm fractures apart from shoulder and knee issues stalling his progress.
But the younger Vunipola believes his steady improvement in recent weeks can help him deliver a good performance in Cardiff.
"I've been okay. I've been out of the international fold on a consistent basis for a while. I think it is important that I try and have a good game on Saturday," he added.
"I do feel there is more to come. It's easy when you are in this environment to have the best foods and drop a little bit of weight... I've dropped a few kilos so hopefully I can produce something close to my best at the weekend."
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Sudipto Ganguly)
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