Yes! Open spots will be filled with Bots, whose stats equal the league's average for the week. For more information on Bots, read this article: How do Bots Work?
Keep in mind, in a Pairs league Matchups are 2-on-2, so the total number of "players" in matchups will be a multiple of 4 (4, 8, 12, etc.). So, if for example, there are 9 people in your Pairs league, the matchups will include 3 Bots (to bring the total number of "players" to 12).
In a Lone Wolf league, Matchups are 1-on-1, so the total number of players must be even. So, at most, there will be 1 Bot in your weekly matchup (unless people leave the league during the week, in which case, they will be backfilled temporarily with Bots)