Niklas, a second-round draft pick from Notre Dame, missed much of the early season with an ankle injury, then sustained a high ankle sprain in Sunday's win over Detroit.
Niklas appeared in seven games with three receptions for 38 yards.
Mulligan, in his sixth NFL season, was released by Chicago on Sept. 16. He appeared in two games, one as a starter for the Bears.
Mulligan, an undrafted free agent from Maine in 2008, has appeared in 67 games, 25 as a starter, with the New York Jets, St. Louis, New England and Chicago.
He has 16 career receptions for 160 yards and two touchdowns.