Ok, lets do it this way.
The Red Sox signed Pablo to a huge contract. At the time he was fat, his production against righties had dipped and they presumably knew his shoulder was at least a bit fucked up. He battled injuries (and weight) his first season of that deal. The second worsened to the point he needed surgery. Since he seemingly was motivated and contrite enough to knock the shit out of rehab and conditioning and looks to be as positive a scenario as could possibly be imagined heading into the season, given his recent history. All the way to the point the team felt it safe to trade the (albeit replacement level) incumbent (that stole his job in spring training last year) and also trade the seeming heir apparent stud prospect.
A brief aside -
It is not past the point of no return on his contract. If you didn't learn this almost exact scenario with the John Lackey experience, I'm not sure what to tell you.
Further, the owners, while generous in payroll are not beyond wanting a return on their money, nor being sick of wasting it. Pablo isn't DD's fault, but JH signed off on it. Just like he did the money being pissed away on Craig and Castillo. They are not just going to pay him to go away or bench him without giving him a chance to earn it. Anyone thinking he's going to play less than 120-130 games this year no matter how well he hits, well, I'd gladly make a side wager over PM with them.
As to Plouffe getting time, let's remember Brock Holt is going to be the primary backup at 2B and 3B, secondary at OF and even 1B. He can emergency sub at SS but it's not ideal, so an addition would be best served to have middle infield capability (Hernandez). Hanley will be playing at 1B a decent portion of the year - they've said they want to use DH as a rotation now and there's no reason to doubt them on that.
The only reason Plouffe would think he'd get more time in Boston than elsewhere is if he has absolutely no market (which is impossible to know; only bored guys bother take the time to dig on guys like this or the reports they have, like the Herald one, are planted) or if his agent genuinely thinks Pablo is going to shit the bed early and often and the Sox are going to cut bait quick, in which case I resort back to him needing a new agent.
I find it very unlikely he gives much of a shit if he signs with a playoff team. His interest is getting some run and getting one last multi year contract from someone. If he signs for that kind of money and plays well, he'd almost definitely be a deadline acquisition and get his playoff experience anyway.
And no, the grass is not greenest in Boston for those things to happen.