Yes, you are correct. The team can theoretically squat on Butler for 2 more seasons (2016 and 2017) for relatively cheap money. As you mention, they'll have to place a high round (likely first) tender on him for the 2017 season, although it's certainly possible that they can reach an agreement when this upcoming year plays out and they have a larger sample size to determine his value.
If you want to take it one step further, they could even place the franchise tag on him for the 2018 season, which will likely be about 15m dollars. Add in the first round tender of a few million or so next year and you still have him locked up for the next 3 years at something near a 6m dollar AAV range--and that's in a WORST case scenario. In that situation, we wouldn't have to concern ourselves with Butler until March 2019.