Haye fights Ruiz at the MEN Arena in Manchester in the first defence of his belt, which he won in November by beating Russian giant Nikolai Valuev.
However, Harrison, who fights Michael Sprott for the vacant European title on April 9 at London’s Alexandra Palace, thinks betting favourite Haye is not in peak condition.
“I watched a training session of David Haye from Sky Sports and his timing is off. He’s also carrying excess weight,” said Harrison.
“I know David very well and mark my words, if he doesn’t blast out Ruiz early, which is possible, then it’ll be a tough night.
“Golden Boy have also signed Ruiz, so they are in with the winner and out with the winner.
“But it’s a funny old game and if it doesn’t go well for David on Saturday night then who knows what the future holds for him.”
But California-based Harrison maintains Haye will not be seen as a true world champion until he has defeated one of the Klitschkos.
“The heavyweight division is desperately looking for marquee names and, no disrespect to David Haye, but no one in the US is talking about David Haye,” added 38-year-old Harrison.
“They’re all talking about the Klitschkos and no one is giving David Haye any play as a world champion so the guys I want to fight are the Klitschkos.
“If you get a chance against David Haye then people will say ‘you now have to beat the Klitschkos’, so if I beat Sprott, I want the Klitschkos.
“The fight with Sprott is obviously huge for me because when I get revenge and lift the title it puts me straight in line for a shot at the world title.
“People are talking about me fighting David Haye at Wembley but, with all due respect to David, outside of the UK no one is talking about him – it’s the Klitschkos they see as the real heavyweight champions so dethroning one of them is what I really want.”
Tickets for the Harrison v Sprott clash for the vacant European Championship at Alexandra Palace on April 9 cost £30 and £60 and are available fromwww.seetickets.com or 0871 230 7148. by Ian McNeilly