Sundowns recruited the 20-year-old on a five-year deal from Al-Ahly last week.
The former Bidvest Wits striker joined Ahly in 2017, but was unable to secure regular first team football in Egypt. He scored one goal and assisted a further five in 15 outings for the eight-time CAF Champions League winners.
'Jingles' has revealed the former Bidvest Wits star has some way to go before he is ready for duty.
"Phaka is in his third week because I have been talking to him for a month or two months ... He did week one and week two with Al-Ahly," Mosimane said.
"In the third week they left for Croatia and they left him behind. He lost two weeks at home and in week four to week five they brought him back and he started training with the team again.
"In my opinion he is 55 to 60 percent fit.
"But we didn't buy Phaka to win the MTN8. We invested in him for the next five years. It's a big investment we did."
Mosimane will work on getting Mahlambi up to speed during the FIFA international break, with Downs next in action against Golden Arrows in the league at Princess Magogo Stadium on September 19.
The Absa Premiership champions, in sixth place on the log with five points from three games, will be eager to reassert their pedigree following their recent elimination from the Champions League and MTN8.