Slimani's goal could serve as a warning to Pirates who travel to Beluizdad for their Caf Champions League Group C opener on Tuesday.
It remains to be seen if Mayo will feature in that match as the former Cape Town City struggles for game time.
Mayo has started just two of Belouizdad's seven Ligue 1 matches this season and is yet to find the back of the net, even in the two Champions League matches he has featured in.