Rio, whose last game for the Hammers was - spookily enough - against Leeds, explains: "It will definitely be an emotional day because I love West Ham and everyone there.
"When I first signed I looked for this fixture against West Ham straight away. It was a long long way away but I thought 'I can't wait for that to come around'.
"It will be nice to see all the boys and everyone behind the scenes; the kit man, Jimmy Frith, and people like that.
"If I lose I will shed a tear! But you have to try not to get too emotional on days like that.
"You have to let the football take precedence and do the stuff on the pitch."
Not that Rio has really cut the umbilical cord; he stays in touch with a large proportion of the West Ham squad and reveals:
"I speak to Frankie all the time, Paul Kitson from time to time, Razor though he's not there any more of course, John Green, Jermain a lot, Izzie Iriekpen, Adam Newton who is on loan at Notts County.
"West Ham is the result I look out for when I get back in the dressing room and I always hope they have done well."
Rio admits his first faltering steps with his new club weren't easy: "It started off a bit difficult and when I first got there I wondered what I had done when we lost 3-0 against Leicester," he admits.
Since then, West Ham have dropped as Leeds have risen and he says:
"It's disappointing because the ideal scenario would have been West Ham winning the cup and Leeds the league or europe.
"But I always want West Ham to do well and I always look for their results."
If he had stayed, would West Ham have had a better season?
"I don't think things like that," he insists. "The management and the board decided to sell me and I agreed to go in the end. It's neither here nor there that I left in that sense."
It is a measure of Rio's modesty that he is genuinely unsure what sort of reaction he will get from the West Ham faithful.
Someone tell him it will be a warm greeting.
"I hope it is but you can't really determine what kind of reception you are going to get from the fans.
"I always gave 110% when I played for West Ham and all the people at West Ham sent me good letters when I left."
But he was quoted as saying he had 'gone stale' at West Ham. Surely some mistake?
"That was out of context; I was asked about my form and I just said I didn't play well for a little while.
"But I wasn't talking about when I left or anything like that, because I thought I wasn't playing too bad at the time. It was a misquote, with all due respect, but people write things and mess about - that's life."
Of course, he has three England team mates to face on Saturday, and he says of Frank, Joe, and Michael: "They are not embarrassing themselves; they are doing really well, although a few results haven't gone so well of late.
"But that's not just a reflection of them, that's the way it goes sometimes - you don't get the rub of the green."
Not, he thinks, that he could help send his old club down if Leeds win. He insists: "West Ham are safe although they will definitely get a win anyway - but hopefully not against us.
"At the most West Ham need a draw but they've got too much quality to get pushed down into the dog fight at the end."
Speaking of life after E13 he says: "The lifestyle after the football is a lot quieter but the professional side is similar. I live out in the sticks and I think it's been better for me to really concentrate on my football."
Does Rio think about what West Ham are doing with the £18m?
"That's down to West Ham," he says diplomatically. "And the directors, and how much they are going to spend; but I'm not one for telling people what to do.
"If the gaffer does get the money to spend then I'm sure he'll spend it very wisely."
Rio has more than just nostalgia to make him follow West Ham's fortunes.
He has family ties still, too: brother Anton is at the club and hoping to follow in his footsteps.
"As long as West Ham stay in the Premiership then he is happy," smiles Ferdinand the elder.
"I always talk to him about how he is doing; he is on the staff next season so the next couple of years will be make or break for him as to whether he makes it or not at West Ham.
"But another Ferdinand coming through at West Ham would be brilliant."
Will Rio be distracted come the whistle for the start of hostilities?
"It'll be good to have all my friends and my family there," he says.
"But there will be mixed emotions about who they want to win - hopefully Leeds will win and West Ham play well!"
At Elland Road Rio has enjoyed a European odyssey with Leeds of course - if rumours of a £125,000 bonus for reaching the final are true he'll be able to buy half of Leeds.
Not, of course, that money motivates him; rather, he is learning new ways of playing the game.
"They play a lot more slowly in Europe and you have to concentrate and shuffle across all the time. It's different football, but you have to get used to it."
And, er, is he going to get the West Ham player of the year award?
"No," he laughs. "Stuart Pearce is going to win that!"
by Khris Raistrick