The Official Online RBS 6 Nations Store is open. The store has everything you need to get behind your team during the RBS 6 Nations, plus the store is now fully stocked with a much wider range of rugby merchandise.
Holmes said: "The key for the Lions is the back-row and and I think the fitness of Martyn Williams is going to be crucial.
"To stand up to them, we need a footballer to link backs and forwards and he has been out of this world."
Williams played seven games for the Lions midweek team in New Zealand in 2005 and four games for the Lions second string in Australia in 2001.
But Holmes believes the 33-year-old openside flanker should be a key member of Lions head coach Ian McGeechan's Test team against the Springboks.
However Holmes warned Williams' Wales and Cardiff Blues back-row partner Andy Powell, who starred in Wales' autumn Test campaign, to improve his discipline.
The 27-year-old announced his arrival on the international stage with a man-of-the-match display against South Africa in November.
And Holmes said: "I thought he was awesome in the autumn series. The only worry is a bit of discipline.
"That top 10 per cent, as All Black coach Graham Henry said, is so vital; that you don't give away penalties in a tight series.
"That's something he's going to have to tighten up, not just for the Lions but for the Six Nations. But his athletic ability was a joy to watch."