Let's get this part out of the way first: Given the foresight of the option to sign Jeff Green or retain Tobias Harris, the Orlando Magic would have been better off keeping Harris. Depending on how you feel, Harris is somewhere between "slightly worse" and "somewhat better" than Green, and if last season's box score stats are an indication, it's probably closer to the latter (listed per 100 possessions):
Last season's numbers (which include Green's time with both the Clippers and Grizzlies) indicate Harris was the better rebounder and a more efficient shot-maker, even from long range. Otherwise, they were matched in just about every category, aside from one other critical metric: Harris is six years younger than Green.
We can tell a similar story with more advanced statistics. Plus-minus stats like RPM and BPM suggest Harris was a net positive on the court (+0.61 and +1.0, respectively), while Green was a net negative (-1.5 and -1.7). Granted, these stats only describe their effectiveness in their respective roles in their respective situations, but I think it says something that Green only dragged down good teams while Harris did his best to lift up a bad one.
So, yeah, it kinda sucks. The Magic are paying Green essentially the same money next season as they would have paid Harris. We pretty much knew it at the time, but this reaffirms that the Harris trade just wasn't a good one for the Magic. The only defense I could make is that the deal brought Ersan Ilyasova to Orlando, and while his on-court contributions were relatively unimportant, his mostly non-guaranteed contract was an essential part of making the Serge Ibaka deal happen...not that everyone agrees the trade was a slam-dunk for the Magic.
That said, to best assess the Green signing, we need to set aside the Harris trade. Perhaps, given the benefit of hindsight, the Magic would have kept Harris and not signed Green, but that's not really a fair evaluation. The Green signing happened in a world without Harris, and we should analyze it as such.
(If you want another perspective on Green's signing, make sure to check out Will's analysis, which especially focuses on Green's role as a veteran and a mentor. I'll be focusing more on his potential on-court contributions.)
My biggest concern with the Green signing is that he doesn't offer any one elite skill that can enhance the team's offense. For example, Jeff Green's not an elite distributor, averaging 2.7 assists per 100 possessions for his career, and about 3 assists over the last three seasons. That's not bad, but not incredible either. Still, even if he's not creating shots for teammates, it'd be alright if he offered strong floor spacing from a forward position...but the last time he shot above 34% from 3 was in 2012. You probably shouldn't count on Green to be a shooting threat at any distance, based on his 33% accuracy on all jump shots last season.
If he doesn't create shots at a high level and doesn't efficiently shoot jumpers, Green could be an elite finisher close to the basket...but he's nothing special there, either, hitting about 51% of his shots in the restricted area compared to the league average of 55.5%.
Unless you think the Magic's greatest need was shots from 12 feet away on the left side of the court, the zone chart doesn't really offer a lot of answers about the holes that Green fills. It'd be swell if Green really could hit 48% of straight-on 3-pointers, not that 25 shots is enough to judge one way or another (if anything, it says more that he took twice as many shots from either the right or left wings, and hit just over half the percentage).
Of course, nobody adds Green to their team because of what he's done before. Orlando is the fifth team to acquire the combo-forward, and the previous four franchises took a shot on Green because of what he could be. For a 29-year old, it's remarkable how much the conversation continues to revolve around his potential, his growth as an NBA player, and fulfilling the career-long vision of a dynamic, athletic, play-making stretch forward.
At this point, we need to give up on the dream of Super Green and live with the one that's existed for eight seasons across four different teams. It's entirely possible that Frank Vogel brings out the best in him, like he did for Lance Stephenson, but his best role for Orlando is someone who can anchor the bench and try to infuse his teammates with whatever veteran wisdom he's picked up along the way.
If anything, my greatest fear is that he absorbs playing time from the Magic's up-and-coming players. Unless Green has the best season of his career, there's no reason he should siphon minutes from either Aaron Gordon or Mario Hezonja. The question of Gordon's playing time already hung above the team with the additions of Ibaka and Bismack Biyombo to the big man rotation, and Green only complicates the matter.
As long as Vogel navigates that issue successfully, Green can be a positive force for the Magic. Green could realistically fill the Jason Smith role, someone who comes off the bench as a solid player whose lack of obvious weaknesses means he can't be easily exploited. The big difference is that while Jason Smith excelled at one skill (mid-range shooting), Jeff Green has approximate knowledge of many things and could flexibly support the Magic in a variety of in-game scenarios.
If Green is faced with a slow-footed bench forward, he has enough skill and speed to put the ball on the floor and make it to the basket. Against teams that leave gaps open on the perimeter, Green can hit enough wide-open shots to maintain the Magic's attack --he shot 35.5% on 3-pointers without defenders within 6 feet, per NBA.com's data. Occasionally, as we well-know from his season-high 30 points against Orlando in January, Green can go off and carry the team.
Of course, his most frustrating quality is his propensity to disappear when the team needs him most. While Green scored 20 or more points in 11 games last season, he scored 10 points or less in 39 of them, including 5 or less in 12 of those games.
That's why it's important we see Green as a role player and not a core player.
If the Magic's season depends on the success or failure of Jeff Green, they've put themselves in a poor position to succeed. Instead, Green's best role is a supportive position, someone to bolster the team when they're low and and coach up the guys behind the scenes. Anything past that is just a bonus.