The L.A. Rams quarterback and his wife got Marley right when Stafford was drafted into the NFL ... and Kelly says she "immediately filled Matthew’s side of the bed."

"I woke up this morning without a face in front of me that I’ve been used to waking up to for the past 13 years," Kelly said.

Kelly noted Marley was their "first baby" before the married couple had four daughters together -- in fact, Marley helped announce one of those pregnancies by holding a sign "Mom's pregnant" back in 2016.

"I know it shouldn’t be a surprise, but I still can’t believe she is gone," Kelly said, "I’m devastated and this house just doesn’t feel as full as it once did."

Stafford also shared a 3-minute tribute video of Marley with the song "The Sun Is Rising" by Britt Nicole playing in the background.

"This song is what I feel like what Marley has said every time I was hurting and crying on her," Stafford said.

"I feel like she’s looking over me saying the same. Miss you sweet girl."