Scorchers announced that Mills, who was part of England's T20 World Cup squad, had withdrawn from the tournament due to a family emergency.
"Home for Christmas after the most horrible 11 days," Mills posted on Instagram. "As we were at the airport about to leave for Australia our daughter suffered what ended up being a stroke.
"She lost complete use of the left side of her body and were told they couldn't predict how much she would regain.
"Despite all of the challenges she faced our little girl has amazed everyone with her recovery to point where we were discharged with her hopping and skipping out of the hospital.
"She has lots of rehab, medication and scans ahead but we are so grateful to be where we are right now.
"Thank you to everyone that's reached out to us. Keep your loved ones close."
They have had to overhaul their overseas signs after all three of the players they brought in from the draft became unavailable. Alongside Mills, Phil Salt was injured during the ODI series against Australia and Laurie Evans' contract was terminated after he failed a dope test.
Faf du Plessis, Adam Lyth and Stephen Eskinazi and currently part of the squad.