The Hopeless Replacement

It's always this way: The team has just made the finals of the big tournament, or the Fake Band has a big gig around the corner. However, the star team member just wrenched his back and is out of the Big Game, or the lead singer has come down with laryngitis.

Cue panic as the group scrambles to find a replacement (and this trope usually has a scene where the group is calling someone with a last name beginning with the letter Z). Down to their last hope, they bring in the love interest of one of the team members. She's the savior of their group...except she stinks...can't score to save her life, can't sing, can barely play tambourine. However, thanks to her romantic arrangements and/or the fact that the team forfeits if they do, she can't be kicked out of the group.

The team trudges on, thinking they'll get creamed in the Big Game or the contest...and they either do, but have fun doing it anyway, or they win in spite of her (or even because of her, bringing the Aesop with it).

Anime and Manga

 * Subverted in Ranma 1/2: after Akane injures herself on the night before the Martial Arts Rhythmic Gymnastics tournament, she despairs at the thought of finding someone who "...is good at martial arts, knows gymnastics, and looks good in a leotard" in such short notice. Everyone immediately shanghais Ranma into service.
 * In the baseball episode of Samurai Champloo, all the members of the team quit before the big game against the Americans, so the ninja coach recruits the main characters. Mugen and Jin are the only useful ones however, as the other players are Fuu, an old man, a dog, and Fuu's pet flying squirrel.
 * In the Dark Tournament arc of Yu Yu Hakusho, circumstances leave both Team Urameshi and Team Toguro with only four of the required five members as they head into the finals, so each team's sponsor (Koenma and Sakyo, respectively) volunteers to fill the missing rank. Possibly Subverted, as neither one are actually combatants, and both freely admit that they will run like hell rather than set foot in the ring.

Film

 * Space Jam: The toons are taken out one by one by one, until finally Michael Jordan's publicist is in the game. When he gets squashed, it looks like they'll have to forfeit...until the miraculous entrance of Bill Murray! Who just fills out the five on the floor as Michael stretches for the winning basket.
 * Played with in The Producers. When the star of Springtime for Hitler has a broken leg, the director fills in at the last minute. He turns the show into a roaring success, when it was supposed to fail, thus being useless by being too good.
 * Not so much "too good" as So Bad Its Good. Turns out the audience couldn't get enough of Camp Gay Hitler.
 * In Shaolin Soccer, the villainous soccer team named...wait for it...Team Evil have used their doping-enhanced strength to injure several of the players on the good-guys team - the game's nearly over, but they're short one goalie, and they have to forfeit if they can't find a replacement. Cue the hero's love-interest entering to demonstrate that her Kung Fu is stronger, and win them the match.

Live Action TV

 * On Family Ties, Alex needs a third for a quiz bowl team, and is forced to use Mallory. He coaches her extensively and she ends up being the star of the team after Alex suffers stage fright.
 * This was subverted in an episode of Clueless, during a big school basketball game when the star player is out. Friends Sean and Murray fill in as replacements, and happen to be really good and beat the other team.
 * In Hollys Heroes, being short a player forced Holly to field the team's statistician on the basketball court, only for her to turn out to be a brilliant goal shooter (but terrible at the rest of the game). She was used a regular player for the rest of the series.
 * Kirk: The bowling team is in the finals, but their best bowler just threw her back out. In comes Kirk's main squeeze, who stinks...until she's given some reverse psychology by Kirk, and pulls out the victory for their team.
 * In the Star Trek Deep Space Nine episode 'Take Me Out to the Holosuite', the Niners bring Rom on as a last-minute substitute after Captain Sisko gets booted from the game, and he makes an accidental bunt that lets Nog score the Niners' only point.

Real Life

 * After the death of Bruce Lee, many actors with similar physical appearances or names popped up in hopes of becoming "The Next Dragon." Jackie Chan was the most popular, due to his movies' content having nothing to do with Bruce Lee, but many at the time still considered him a hopeless replacement.
 * It took some time before he was able to make those movies; that his earliest leading roles in China were direct attempts to make a formulaic "Bruce Lee movie", and that he was unable to duplicate the actor's intensity and (in-character) fury had a lot to do with it.
 * Happens often in team sports. Brutal in international tournaments, where a country might only have one or two capable players in an important role. Comes into play in soccer games: if the goalkeeper is sent off and there are no others available, an outfield player will be moved back. If the player actually succeeds in the position, fans will invariably remember it as a Crowning Moment of Awesome.

Western Animation

 * Parodied on Futurama, "Time Keeps On Slipping". Fry volunteers to sub in a basketball game so the team won't forfeit. The team is leading by a large margin and the game is nearly over, but Fry costs them the game anyway, and he doesn't understand how that happened (see episode title).
 * A good decade or two before that, guess who did something similar in a short in the The Charlie Brown and Snoopy Show.
 * Subverted (or inverted? Or just parodied?) in an episode of South Park: The boys need to win a dance contest, and try to recruit Butters, who's a world class tapdancer. He refuses because he accidentally killed 15 people last time he danced, so instead they recruit a dancing duck. Then, in the climax, the duck is injured, and Butters subs in for him.