When the original version of “The Manchurian Candidate” was released back in 1962, one acid-tongued critic wrote that Hollywood had finally found a role that Laurence Harvey could play–that of a brainwashed zombie. If one wanted to be really cruel, he could similarly suggest that the perfect part has now been found for Johnny Knoxville–a mentally challenged fellow. But there would be two problems with that. One’s that Steve Barker, the character Knoxville plays in “The Ringer,” isn’t really challenged in that way: he’s merely pretending to be so that he can get into the Special Olympics and allow his sleazy Uncle Gary (Brian Cox) to bet on the competition and not only erase the huge debt he owes his bookie but get Steve the funds he needs to pay for digit-reattachment surgery for a fired janitor at the company where he works, whom he’d arranged to take over the post of gardener at his apartment complex (though without health insurance, unfortunately). The second problem is that Knoxville’s mentally-challenged routine is pretty awful, especially compared to the actors (a few really challenged, most not) by whom he’s surrounded. They’re good; he’s not. (In fact, when, in his first scene–before Steve asks his boss for a promotion–Knoxville repeats, Stuart Smiley-style, “I’m a winner, I can do it,” you’re already tempted to reply, “No, you’re not, and you can’t.”)

“The Ringer” is a Farrelly brothers production (though it’s directed by Barry W. Blaustein), so the recipe is fairly standard. The premise threatens an orgy of tastelessness, but the result is actually rather sweet, because those whom you fear will be made the butts of the jokes are instead treated as really heroic, as well as clever and funny, and the fraud–who’s conflicted to begin with–comes to consider them not only his friends but in many ways his superiors. In fact, the movie could stand more of its least PC element, the grossly insensitive Gary, whom Cox plays with delightful abandon, but who disappears for far too long in the mid-section.

No, the problem with this movie isn’t that it’s tasteless–because it’s not–but that it’s flaccidly paced and overly predictable, and burdened with Knoxville’s stiff, charmless performance–even his numerous pratfalls don’t satisfy. Even worse is the addition of a romantic subplot in which Steve falls head over heels for a Special Olympics volunteer, the lovely Lynn (Katherine Heigl), whose boyfriend (Zen Gesner) naturally turns out to be a handsome cad. And when it comes time to wrap everything up, screenwriter Ricky Blitt (“The Family Guy”) is so desperate to tie up every loose end in a fashion that allows for everybody to be happy that he throws anything he can think of on the screen, from a mawkish public redemption scene to a big dance number.

“The Ringer” has its heart in the right place, and it’s reasonably well made from the technical perspective, with Mark Irwin’s cinematography and Mark Mothersbaugh’s score (which makes ample use of Elmer Bernstein’s famous theme from “The Magnificent Seven”) standing out. But while it’s certainly not as bad as you might expect, neither is it as good as you’d like.