Lynch has already been announced as part of a Triple Threat Match against Charlotte Flair and Asuka at TLC, which hits on December 16th. Thing is according to a recent report by the Wrestling Observer she still hasn't been officially cleared for action, and the pay-per-view is just a few weeks away. She did recently participate in some in-ring action as part of WWE's Tribute to the Troops, but it was in a tag match with Charlotte Flair, so the heavy bumps were probably taken by her partner as a precaution (SEScoops).
The reason why she hasn't been cleared for in-ring competition actually might suppose you. According to the report Lynch has been cleared of the concussion from the Nia Jax punch during the Monday Night Raw invasion before Survivor Series, so that's good news, as concussions are nothing to mess with. Thing is she has evidently not been cleared of the nasal fracture that came as a result of the punch, and that's what everyone is waiting on before she can truly go all in at TLC.
The good news is she is expected to be cleared in time for the pay-per-view, which is why WWE booked the match in the first place, though they could always call an audible if they absolutely have to, much like they had to do at Survivor Series when Charlotte was placed in Lynch's spot in the Ronda Rousey match. They had to do the same thing at Evolution when Alexa Bliss suffered a concussion before that pay-per-view, subbing in Alicia Fox in the Mikie James vs Lita and Trish Status Tag Match.
You can check out the full card for TLC below, though things could always change before the show airs.
Triple Threat TLC Match: SmackDown Women's Champion Becky Lynch vs Charlotte Flair vs Asuka
WWE Champion Daniel Bryan vs AJ Styles
Intercontinental Champion Seth Rollins vs Dean Ambrose
Raw Women's Champion Ronda Rousey vs Nia Jax
Tables, Ladders & Chairs Match: Braun Strowman vs Baron Corbin
Finn Balor vs Drew McIntyre
Elias vs Bobby Lashley
SmackDown Tag Team Tritle Triple Threat Match: The Bar vs The New Day vs The Usos
You can catch WWE TLC live on the WWE Network on December 16th.