It would be unwise to trade Teheran now. There are many FA options, and his stock is a bit low after a disappointing season. Trading him now would be selling low.
Selling Miller now would be selling high, but the myriad FA options also makes it unwise to trade him. I would be looking to extend him and buy out 1-2 of his FA years. He will likely make around $25m over his 3 arb years, so something like 4/40 or 5/55 would make sense. Maybe 3/25 with an option for $15m and a $2m buyout.
Part of the reason Coppy claims they traded Simmons was due to the high demand and low supply of the SS market. Hopefully he uses the same type of logical regarding the pitching market and realizes this is not the time to deal either Miller or Teheran.
It all just depends on the return. If someone is willing to give you a return for Teheran that would match his peak value, you do it.