Shortly after Tuesday's surprising move by Detroit, Slay was transparent about his anger in a series of tweets.
Asked if he was still agitated over losing Diggs to the Seattle Seahawks, Slay responded, "Hell yeah," according to MLive's Kyle Meinke.
The two-time Pro Bowler then refused to say he wanted to remain in Detroit ahead of Tuesday's trade deadline, saying he "wouldn't personally care" if he was traded.
"I'm here to play ball," Slay said. "It don't matter if I'm here or not. That's what I want to do, I just want to play ball."
Slay went on to criticize the Lions and the NFL as a whole for the lack of loyalty to players.
"At the end of day, I just see it as there's no loyalty to nothing," Slay said. "No matter how much you put in, they feel like it's a little different, they can get rid of you. So I just play ball."
Slay has been one of the Lions' best defensive players since being taken in the second round of the 2013 draft. He was named a first-team All-Pro in 2017 after securing eight interceptions, tied for the most in the NFL that season.