Dave Meltzer reported on Wrestling Observer Radio that the angle with Nikki and Natalya was the plan from the start and Nikki is not injured. That angle had been planned ever since officials designed the match several weeks back.
Meltzer speculated that this is being done to set up a Nikki vs. Natalya feud.
On a related note, there was some speculation on SmackDown Women's Champion Becky Lynch eliminating Nia Jax by submission during the same match. That was always the plan, according to Meltzer, but apparently the submission was supposed to come a few minutes earlier, right after Becky and Alexa Bliss double teamed Nia. After the double team, Becky dropkicked Nia while she was holding Bliss and then tangled with Bliss some more before making another pin attempt and then getting the Disarm-Her applied.