Celek, 26, signed on with the 49ers as an undrafted free agent out of Michigan State back in 2012. He was set to be a restricted free agent in the coming days.
Last year, he was limited to appearing in just three games and catching two passes for 53 yards receiving and no touchdowns. According to Pro Football Focus, Celek played just 35 snaps for the 49ers in 2014.
Nelson, 28, originally signed on with the Saints as an undrafted free agent out of New Mexico State back in 2011. He appeared in all 16 games for the 49ers in 2014.