Matt Miazga is still a Chelsea player at the moment. Despite rumors and reports of various moves (including a loan to Eintracht Frankfurt), nothing has come to fruition.
However, buried in a story by The Guardian regarding Younes Kaboul potentially leaving Sunderland for Watford is a tidbit on the young American center back:
The Chelsea defender Matt Miazga, meanwhile, has been offered to Sunderland on a season-long loan.
Kaboul leaving the club could potentially create an opening for Miazga to get playing time. However, the Black Cats also recently brought in center backs Paddy McNair and Papy Djilobodji.