Let say that someone is in master, and they got decayed back to 3000. They decided to play again, I assumed they are versus against other masters even with a diamond rank.
Let say that they have a lose streak, and their diamond rank dropped down to plat. What rank the mmr gonna match them against?
It will still put you against masters. You also only lose like 3-5 per loss and win 50-70, depending how far you are from your actual SR. So you can lose 10 in a row and still be in the 2900s.
Basically what DongQuixote said. In more detail, see 3000+ Skill Rating Data and Analysis (now including DCs), and note that you are always matched by your hidden MMR, which continues to move normally as you play games, but does not move when you decay.