Second string and Underling

Second string noun - A squad of players that are available either individually or as a team to relieve or replace the players who started the game.

Underling and second string are semantically related. In some cases you can use "Underling" instead a noun phrase "Second string".

Underling noun - One who is of lower rank and typically under the authority of another.

Second string and underling are semantically related. Sometimes you can use "Second string" instead a noun "Underling".