Is there a word for an inferior substitute?

I was wondering if there is a word in English to describe a person fulfilling another's role, albeit not as proficient.

There is a phrase, a poor second that conveys some aspect of this concept

a long way behind the winner in a race, competition etc. My horse came in a poor second

Also consider the B-Team

A second-rate problem-solving team sent in because more qualified personnel (the "A-Team") are unavailable or unwilling to deal with a given problem. This is often because the A-Team sees said problem as being beneath them or unworthy of their talents.

Also consider some of the following.
• understudy, “a performer who understudies; a standby”. One sense of the verb understudy is “to study or know a role to such an extent as to be able to replace the normal performer when required”. Understudy usually refers to a substitute less capable than the normal performer, but sometimes the understudy is quite capable.
• stand-in (already mentioned in other answers) means “A person of similar size and shape to an actor that "stands-in" for the actor during the lengthy process of setting up a shot, who, unlike a double, does not appear in the film” and ordinarily is a person less talented than the actual performer.
• deputy, “One appointed as the substitute of another, and empowered to act for him, in his name or his behalf; a substitute in office...”
• substitute, “A replacement or stand-in for something that achieves a similar result or purpose” and “A player who is available to replace another if the need arises, and who may or may not actually do so”
• ersatz, (previously mentioned in comments) “Something made in imitation; an effigy or substitute”
• knockoff, “An imitation of something, particularly a well-known product, usually lower in quality and price than the original”
• wannabe, “Someone who wishes to be or do something, but lacks the qualifications or talent; an overeager amateur; an aspirant”; also would be

Surrogate.

Something less negatively sounding might perhaps be just substitute.