To all those out there worried about this signing and confused about why Danny would do it..
There is every chance that Ainge tried, but was not convinced that he would have a shot at getting a top (or even very good) free agent at comfortable price. The deals that players were asking for may have all been deals that would have required a huge long-term investment by Danny, with a high risk of not paying off (e.g. Harris on $60M / 4). It may be that he was not convinced that making those kind of large, long term commitments were too risky for the type of players that were available.
BUT
After making the playoffs last year, Boston have shifted from rebuilding mode to 'win now' mode, so they do still want to add pieces that they feel can help them win games.
These deals (if the reports are true) fit very nicely in to that category. Amir Johnson gives us a legitimate improvement at the PF spot (in a bigger way than people probably realise) over the next year, and if the second year is guaranteed then it puts Ainge to remain in a great position when the big free agent spree hits after this season by either:
a) Including Johnson's $12M non-guaranteed deal in a trade
b) Declining the second year of the deal and having $12M in immediate cap space
So if anything that puts us in an even better position then we're in now because currently we don't have pure cap space, we only have TPE's that can be renounced or used.
So if you don't see any free agent that's available to you, that you feel is willing to take a short term contract, and/or that you feel is worth committing to with a long term contract...then signing a guy like Amir with only a one year commitment makes perfect sense.
Same with Jerebko - he was great last year and showed real flashes of potential. Now we get to trial him for one more year to see if it was just a fluke or if he has some real game in him. If he does, we get him for an extra year - if he doesn't we can just let him go.
A 2 year deal with a second year non-guaranteed is the perfect type of contract for players like this, and for filling roles like this.