Heath said Hlongwane has a "little bit of tendinitis, we think" and is sore just below his knee after he left Sunday's 2-0 victory at New York City FC in the 38th minute.
He was replaced by recently reacquired central midfielder Jan Gregus, who scored the eventual winning goal in the 53rd minute.
The 15 goals Hlongwane has scored this season in all competitions are the most in club history. He scored seven in Leagues Cup play — second only to superstar Lionel Messi's nine — as well as six more in MLS play and two in U.S. Open Cup games.
All of Hlongwane's seven came within the 18-yard box.
"Obviously, he's a top goal scorer," Heath said. "He's a threat to score in every game, and this weekend he had a couple of great chances again. He has been working really hard on both sides of the ball. He's becoming a really important piece for us. Obviously, it's important for us to keep him fit and healthy."
Hlongwane came out for warmups during Tuesday's training in Blaine but was sent back inside to work individually because he experienced some knee soreness.
Heath said Hlongwane will work inside the next few days.
"We'll have a better idea later in the week," Heath said.
Will Reynoso be ready?
Heath called star midfielder Emanuel Reynoso's ankle a "little bit sore" and "not 100 percent" after he was removed from Sunday's game in the 68th minute.
Reynoso participated little in Tuesday's training, but Heath expects "he'll feel a lot better by the end of the week."
Sang-Bin Jeong subbed into the game for Reynoso partly because Reynoso's ankle was hurting, partly because Reynoso picked up a yellow card six minutes earlier. Heath didn't want Reynoso getting a second and thus a red card and ejection.
Take it easy
"They've played a lot of football," Heath said after training. "Today was more of a regeneration day for them. They'll get back tomorrow with the full group."