Brockie traded SuperSport for Sundowns earlier this month, having plundered 54 goals in three years with Matsatsantsa - three goals short of the club record held by the late Abram Raselemane.
The New Zealand striker was also key to United's run to the CAF Confederation Cup final this season, top scoring in the competition with 10 strikes.
"'Brocks', at the end of the day, he was our main man," said Pieterse.
"He was quality and despite him scoring or not scoring, at the end of the day he was our focal point.
"He was the guy who used to win us games in the end. Losing a player like that is never easy to replace.
"So obviously the club will obviously bring in somebody to replace him."
Matsatsantsa have only won twice in 12 matches since they were denied continental honours by Congolese giants TP Mazembe in November.
Pieterse concedes United, who let in a last-gasp equaliser against Polokwane City at Lucas Moripe Stadium on Monday, are in a "dog fight" to resurrect their season but has faith they will get the job done.
"With the players we have, we can definitely do it. We know it won't be easy. It will be a dog fight and that is what we are preparing for."
Eric Tinkler's side are five points above the drop zone after 18 matches, and next host relegation-threatened Premiership champions Bidvest Wits on Saturday night.
Tinkler will be without suspended centre-back Morgan Gould on the weekend.