His decision came before Gronkowski's announcement, as the uncertainty meant there was a "clearer path to a top role" with New Orleans.
He should still be a big part of the Saints offense while catching passes from future Hall of Famer Drew Brees, so he likely won't regret his decision. Still, this could end up being a big loss for the Patriots.
Cook finished last season with 68 catches for 896 yards and six touchdowns, enough to earn his first career Pro Bowl selection.
While he could have slid right into the offense to replace Gronkowski, the team now has no one obvious on the roster to fill the role.
Jacob Hollister, Ryan Izzo and Stephen Anderson were all on the roster during the past year, but only Hollister got any action and totaled just four catches.
The team recently added Matt LaCosse from the Denver Broncos, and he would presumably move to the top of the depth chart. However, he still only had 24 receptions in 2018 after catching just three balls in his first two seasons.
This could mean tight end becomes a high priority for New England in the upcoming NFL draft.