Celtic tried unsuccessfully to recruit the former Orlando Pirates man at the beginning of the season.
However, Phunya Sele Sele have since renewed their pursuit of Mthembu, who is yet to play for Amakhosi in 2016/17.
"We are talking to Chiefs," Celtic CEO Khumbulani Konco tells KickOff.com.
"We are interested to bring him in on loan."
Mthembu scored six goals in 12 starts for Siwelele on a loan spell from Pirates in 2010/11.
The domestic transfer window opened on Tuesday and will close at midnight on January 31.