While it's similar to a minimum deal, Boston actually inked Johnson to a four-year pact using part of what remains of the midlevel exception while signing him for the remainder of the season and three additional years.
The breakdown of salary:
2016-17: $1.1 million
The final three years are all nonguaranteed and the final two seasons have fully-guaranteed trigger dates (Sept. 1 in 2015 and 2016). Boston also holds a team option on that 2016-17 season.
The Celtics are protecting themselves a bit with a lengthy deal, keeping Johnson under their control long term should he emerge as a viable role player. And if their roster fills up and he's not part of their future, Boston has the potential to cut bait or use him as a trade asset.
Boston previously used a portion of its midlevel exception to sign Vitor Faverani and Phil Pressey to similarly structured low-risk multiyear pacts (both Pressey and Faverani signed three years deals, with qualifying offers looming in 2016-17 that could keep them here even longer).