Say you have a soccer team from a small high school, and only 14 students sign up to play. Now suppose several students twist their ankle or for some other reason aren't able to play. Now you have to bring a weak player off the bench for the whole game. What's this weak player called? He's only there to avoid forfeiting the game for a shortage of players. Is there a word for this?
One possible term to describe this phenomenon is a "Stand-in." The usage for this term is more common in eSports than traditional sports, but it nonetheless holds a similar connotation.
Historically, the term has been used in theater and film to describe the actor or actress that substitutes-in for the main role they represent. The reasoning for this position goes something like this, "Professional lighting and camera setup are always done manually and can be extremely time-consuming and tedious. Actors strongly prefer to be elsewhere during that time."
Hence the use of the "Stand-in"
In eSports and Traditional Sports Terminology
"Stand-in" within a sports context, has become synonymous to someone who simply fills-in for the participant so a forfeit does not occur. Examples of this are in the video game "CS:GO:"
An article in Dexerto by Ross Deason makes use of the term to describe this situation:
"Luis ‘peacemaker’ Tadeu will compete at the ELEAGUE Boston 2018 Major as a stand-in for his former team, TyLoo."
Luis Tadeu essentially subbed in because his former teammate, Hansel ‘BnTeT’ Ferdinand, was unable to compete due to visa issues, and would've caused the team to be unable to compete; thus, the aquistion of "stand-in" Luis Tadeu, a coach at the time.
An article in SeatGeek by Bob Knudsen utilizes the term, and seemingly defines it, in a similar fashion:
"In an ideal situation, most professional sports teams have a deep enough bench to cover when a player gets injured or is otherwise unable to play. Backup quarterbacks, point guards, pitchers–you name it, teams have it. But occasionally a team simply runs out of players who are normally able to fill in for a position, leading to sometimes great and sometimes terrible results. Here are the most memorable stand-in performances by players who had no business being out there in the first place."
The article then goes on to detail 4 performances of a similar character, in particular, an instance of a youth hockey coach, Eric Semborski, getting brought up to play goalie for the Chicago Blackhawks, so they could fill-in the goalie position potentially.