I don't think the writing was on the wall, Both Hartline and Gibson lost snaps and targets, so there was opportunity for Matthews.
Kid just either doesn't study or can't pick up NFL level offenses.
I noticed this was a problem when every time he got on the field they had to pull him off after every play and explain how to read the defense and what he was suppose to do. This happened when Gibson and Hartline both were injured, and it was the end if the season.
He still had no clue what he was doing.
I saw the same thing under lazors offense, he would make a great catch, and than back to the sidelines to get coached up.