The Union will not practice Thursday, but Mbolhi is expected to go through an individual workout.
MacMath had started every MLS game for the Union in the last two years until he gave way to Andre Blake during a 2-0 loss in Houston on Friday.
"I was a little bit surprised, a little bit disappointed," MacMath said. "Obviously every player wants to play as long as they are healthy."
MacMath said he understood the decision to play Blake.
"It was a good opportunity for Andre, and he did very well," MacMath said. "And I am looking forward to hopefully playing this weekend."
Union midfielder Cristhian Maidana, who missed the last four MLS games with a hamstring injury but played 23 minutes in the Aug. 12 U.S. Open Cup win at Dallas, is practicing with the team.
"The medical staff wasn't happy with [the decision to play him in] Dallas and thought we might have forced him a little too early," Curtin said. "He got an injection in the hamstring and has been on a very good plan."
Defender Fabinho, who played the entire game against Houston, was scheduled to have an MRI exam on his knee. Besides that, Curtin said the team is relatively healthy.
The Union (6-9-9, 27 points) are one point out of the fifth and final Eastern Conference playoff spot. San Jose took a 6-9-6 record into Wednesday's game at Seattle.
More bread from Bimbo
The Union renewed their multiyear partnership agreement with the team's jersey and team-apparel sponsor, Bimbo Bakeries USA. While no terms of the deal were released, Sports Business Daily said it is for five years and $11 million.