It is likely that Mbolhi, who had 11 saves in Algeria's second-round, 2-1 loss in extra time to eventual World Cup champion Germany, will not wait long to become the Union's keeper.
After Friday, the Union will host Montreal on Aug. 9, and that could be his first appearance, although Curtin wouldn't be specific.
Goalie was already considered a strength before Mbolhi's arrival. MacMath, who turns 23 next Thursday, has started every Union MLS game over the last two seasons. The backup is Andre Blake, 23, who was the No. 1 overall selection in this year's MLS SuperDraft.
Curtin did say that MacMath would start Friday and that Blake would be the backup.
"Rais will be back here doing some fitness," Curtin said Wednesday at a news conference to introduce the new keeper. "We will have dialogue behind the scenes with myself and Rais, but when he is ready to go, he is our starter."
The Union won't have all three on the roster for very long.
"The object is not to have them sit for several years or anything like that," Curtin said of MacMath and Blake. "There are a million different variables, and a plan is in the works now."
MacMath was unavailable for comment, but Curtin said the two have talked. It's safe to say that he wasn't doing cartwheels about losing his job.
"It was a hard discussion," Curtin said.
The same was the case with Blake, who has played in friendlies and the U.S. Open Cup, a non-MLS tournament, for the Union.
Mbolhi has played for Algeria in the last two World Cups. Before joining the Union, he played for CSKA Sofia, one of the top teams in the Bulgarian A Football Group.
"It's a big day," Mbolhi said. "I am really proud to be part of the Union today."
The 6-foot-3, 176-pound Mbolhi attended a Union game a few weeks ago and said he loved PPL Park and the atmosphere. He will look to feed off his World Cup experiences.
"It was a great experience, and I also learned a lot about playing at the very highest level," he said. "I think I grew as a man and as a goalkeeper by playing in the two World Cups."
The Union entered Wednesday just one point out of a fifth and final Eastern Conference playoff spot with 13 games to go.
"This puts us right in the thick of things in the Eastern Conference and [to] make a run in these final 13 games," Curtin said about the addition of Mbolhi.
In a related move, the Union waived goalie Brian Holt.