Niemi, 34, had a 7.49 goals-against average and a .797 save percentage. That includes allowing seven goals in a loss to the Tampa Bay Lightning on Saturday.
He gave up four goals in under 10 minutes during the Penguins' 10-1 drubbing by the Chicago Blackhawks on Oct. 5.
"[Niemi] just drew the bad hand for three games," general manager Jim Rutherford told reporters, according to the Pittsburgh Post-Gazette. "He had the back-to-back games and we didn't do a lot to help him, so I feel bad for him."
Niemi had signed a one-year, $700,000 contract with the Penguins in the offseason. Niemi, who won a Stanley Cup with the Blackhawks in 2010, had been with the Dallas Stars the previous two seasons before they bought out the final year of his contract.
Pittsburgh called up rookie goaltender Casey DeSmith from its AHL affiliate in Wilkes Barre/Scranton.
DeSmith and Tristan Jarry will share backup goaltending duties and will be evaluated on a "week-to-week basis," according to Rutherford.
The Penguins also announced that forward Carter Rowney had been placed on injured reserve after he left Saturday's game with an apparent hand injury.
The Associated Press contributed to this report.