There is a "very, very small percentage chance" that Apple will unveil a blockbuster product at its media event on Tuesday, and that's the "big reason" investors should take profits on its stock now, a longtime Apple analyst told CNBC on Thursday.
Apple has set the bar so high with the massive success of the iPhone and iPad that it must deliver a game-changing product to move the stock, Andy Hargreaves, a senior research analyst at Pacific Crest Securities, said on "Squawk on the Street."
"It's really, really hard to get anything to be as successful as iPhone and iPad. I mean those are basically the two most successful consumer electronics products in history," he said. "And to make a dent with a wearables when every product that has come so far would have been a colossal failure on Apple's scale is just a tough hurdle."
Barring company executives and a few employees, nobody knows exactly what the technology giant will announce, but that hasn't stopped Apple observers from speculating.
Many think Apple will reveal the new iPhone 6 with both 4.7-inch and 5.5-inch screens—bigger than the 4-inch screen on the iPhone 5s and 5c. Others think Apple could unveil wearable technology, though Hargreaves is "skeptical" it would make waves. And the iPhone 6 sales are already priced into the stock, he added.
"Once you get through the iPhone 6 cycle you have really difficult comps, and you're back to dealing with saturated markets," he said. "That means growth slows, the multiple contracts, and it's hard to get the stock higher."
So whatever Apple comes up with, it should be something revolutionary, yet practical, Hargreaves said.
"They're going to have to show something that, I think, changes the world to be quite frank. It's going to have to be not only massively valuable to consumers on an everyday basis, but it has to be something that a lot of people can buy. We'll see," Hargreaves said. "They'll have to come up with something that I haven't thought of for sure."
Hargreaves thinks a lot about Apple, too. He's covered the stock for roughly eight years, and yet has only changed his rating twice. If Apple disappoints on Tuesday, though, he plans to issue a downgrade.
—By CNBC's Drew Sandholm