You can continue to make changes to your fantasy roster, including readding this player, until rosters lock in:
Your Fantasy Roster for
How Automatic Roster Carry Over Works
If your roster is completely empty at the time of scoring a match day (and automatic roster carry over is enabled for the match day), your roster from the previous match day will automatically be copied over and used for scoring.
FAQ
Q: What happens if the roster automatically carried over is not valid?
A: Players that make your primary roster invalid will be removed. If a set of players are not valid because the maximum players per a team limit is exceeded, the highest scoring players in the set will be removed, so that the players remaining in the set no longer exceed the maximum players per a team limit. The highest scoring players are removed (as opposed to say the lowest scoring players) to eliminate the ability to exploit this system.
Q: If I added players to my roster and then removed them all, will automatic roster carry over still take place?
A: The state of your roster at the time of scoring is the only state that is considered. From the site's perceptive, adding players to your roster and then removing them is the same as if those players had never been added to your roster in the first place.