Coetzee has been an important member of Sundowns this season, with Mngqithi preferring to play him as a six ahead of Teboho Mokoena.
This will be a big blow for the Chloorkop-based giants, as the 53-year-old mentor would have wanted Coetzee available ahead of Champions League group-stage matches.
As things stand, Masandawana have nine games to play in all competitions until the end of the year. Amongst the upcoming fixtures, Downs will face Congolese club Maniema Union on November 26, in the Champions League.