Jason Terry said yesterday that he would still give the Dallas Mavericks one last chance to counter the three-year, $15.7 million deal he received from the Boston Celtics, but, well, the Mavericks have already said goodbye and thank you, and several news outlets have reported that Terry has verbally accepted Boston’s offer (since he can’t officially sign any paperwork until July 11, when the free agent moratorium ends).

Now the old Celtics are even older, with a sixth man who will be 35 years old when the regular season starts. Terry has played more than 34,000 minutes in  his career, which makes him a rookie relative to Kevin Garnett but in need of a walking cane relative to most folks. Consider: Terry has played just 200 fewer minutes than Larry Bird did during his entire career. His signing does not come without risks, such as “ONE OF THESE DAYS, ONE OF THESE OLD MOTHERFUCKERS WEARING GREEN IS GOING TO GET SIGNIFICANTLY WORSE.” On the flip side, Terry has never missed more than eight games in a season and has missed just 16 games total over his last five seasons. He’s been as durable as the War Horse (yes, I’m ashamed I watched that) and approximately as consistent: Terry has averaged somewhere between 17.1 and 20.9 points per 36 minutes for each of the past seven years. His PER (15.7 this past season) has slipped from 19.3 in his 2008-09 Sixth Man of the Year campaign, but it’s just a tick worse than it was in 2009-10 (16.5) and hasn’t been at or below league average since his rookie season.

This is not to say that Terry arrives in Boston without risk. He will turn 37 years old before his contract expires, thanks to the third year Danny Ainge had to give him in order to swipe him away from Dallas (which reportedly offered Terry more money but only two years). JET, like much of Boston’s nucleus, is capable of showing up one season as a shell of the player he once was. That’s what old players often do. They feign an ability to pause time until one day, time hits fast forward.

While he’s still pausing time, Terry will provide a lot of what Boston needs. He’s a reasonably efficient, high-scorer for a team that needs exactly that (Boston’s offense was ranked 24th in efficiency last season, sandwiched somewhere between the New Jersey Nets, Toronto Raptors and the moldy cheese I threw out last night). He drilled more than two 3-pointers per game, which will significantly aid Boston’s offensive attack, which last year consisted of far too many midrange jumpers and not enough triples or free throws, the two most efficient ways to score. He loves coming off the bench, and he’ll instantly become Boston’s best second-unit playmaker, regardless of whether Jeff Green or Ray Allen return. When you compare Terry to Boston’s last three mid-level signings — Rasheed Wallace, Jermaine O’Neal and Chris Wilcox — well, there is no comparison, because if Terry doesn’t outproduce all three of them I’ll watch the movie Gigli on loop for four straight months.

But even if Terry is great, the C’s pursuit of Allen probably hit a road block. The Celtics still want Allen. Badly, if you believe the reports. If he does return, a bench of Terry-Allen-Green-Sullinger-Center X is enough to give Erik Spoelstra a few additional nightmares this summer. But it seems like a big if.

I admittedly have no inside information about Allen’s decision, but I’ll eat a pound of dirt if the Terry signing helped Boston’s pitch. Allen was upset last season about losing his starting role. Now he’s supposed to share shooting guard minutes with both Terry and Avery Bradley, who both obviously deserve a lot of playing time? (Note: I know Bradley might be out until December, according to Rivers. Still.) Terry and Bradley can both play a little point guard, but there are only so many minutes available if the backcourt consists of Bradley, Terry, Allen and Rajon Rondo. If Bradley receives the time he probably deserves, either Terry or Allen might become disgruntled. If Allen and Terry get most of those minutes, it would stunt Bradley’s development (obviously, as he showed last season, he needs to play). What might make more sense is using Allen in a sign-and-trade, preferably for a big man. But finding a decent return from one of Allen’s suitors isn’t easy. Believe me, I tried. For 45 minutes. And the best fetch I mustered was Kenyon Martin, not exactly an All-Star at this stage of his career.

Regardless of whether they can still lure Allen, the Celtics have improved by adding Terry. Does he make them a contender? Zach Lowe has a nice, tempered take which he concludes as follows:

If that defense can withstand the aging process for one more year, and if Terry can spur the offense, and if the starting lineup with Bradley at shooting guard is really as efficient as it looked last season, and if everyone stays healthy … then the Celtics have a shot to do something big. But that is a lot of “ifs,” and the teams that play for a title in mid-June typically don’t have so many. But this team’s worst-case scenario just got better, and the odds on its best-case scenario just improved a bit.

The Celtics had planned a rebuild after the 2012 season. But then the old guys showed serious life in the 2012 playoffs, the free agent class became weak, and Danny Ainge decided he had no choice but to extend the Kevin Garnett era. Ainge isn’t entirely focused on winning now, but he believes the Celtics might be able to contend by bringing back the same nucleus with a few peripheral tweaks.

Terry is one of those peripheral tweaks, and a significant one. With the Miami Heat and Oklahoma City Thunder likely to remain as powerful as ever, the Celtics still don’t have much (any?) margin of error in the upcoming season. But by signing Terry, their slim chances just became a little less so.