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
Something less negatively sounding might perhaps be just substitute.
You could call the person a second-stringer. It's a term borrowed from athletics but people use it in non-athletic contexts. Second string refers to players who are used as alternates (for sports that use alternates). There's less of a negative connotation. They can be good players, but usually not as good as the first string.
stopgap: someone or something that is intended to be used for a short time and then replaced by someone or something better
makeshift: a usually crude and temporary expedient: substitute
quick-fix: an expedient usually temporary or inadequate solution to a problem
fill-in: someone who takes the place of another person who is away for a short time: someone who fills in for someone else
pinch hitter: any substitute for anything, esp. in an emergency
body double: a movie actor that substitutes for a leading performer, especially in distance shots or scenes not involving the face, such as close-ups of a portion of the body