A baseball roster is an organic entity, shifting and reshaping throughout a season.

“One of the difficulties from the GM spot is that you address a weakness and something else immediately becomes your weakness,” Red Sox president of baseball operations Dave Dombrowski told me. “There is no such thing as a perfect club.”

Only the striving for perfection. Which never ends. Which only reaches greater intensity at this time of year, more so for the Yankees and Red Sox when they are both this good and in each other’s way.

The Rivalry renewed in its most familiar form Friday night — on the field. But for Dombrowski and Brian Cashman, the game is endless and relentless, not constrained by nine innings.

“Whenever your team is good enough and [has] proven it is good enough, the effort in this job is to find ways to get to even another level, to reinforce and improve,” Cashman said.

As much as any time in the history of this New England-New York feud, the ability to find extra wins is vital. The teams arrived at this three-game series with the majors’ two best winning percentages — Boston leading the AL East by a single game. Only one can win the division. The other likely will host a one-game sudden death Oct. 3 — postseason survival under immediate assault.

So, both teams continue to reconfigure their rosters, because the difference between unearthing that extra win or two is likely to be the difference between a best-of-five Division Series and the Oct. 3 jeopardy.

Both teams added righty bats for this series — and beyond.

The Red Sox on Thursday obtained Steve Pearce from the Blue Jays to further address a deficiency against lefty pitching. Dombrowski said the Red Sox were working several avenues and it was merely coincidence the deal with Toronto was finalized in time for Pearce to be in the starting lineup to face a lefty he has good stats against, CC Sabathia.

The timing of Brandon Drury’s promotion definitely had to do with Boston starting three lefties over the weekend — Eduardo Rodriguez, Chris Sale and David Price. But Aaron Boone said Drury is here to stay, which will probably mean assuming the backup first base and third base at-bats that have been going to the struggling Neil Walker.

Next, the Rivals believe they will get back rehabbing pitchers. Dombrowski said he was encouraged by reports on reliever Tyler Thornburg and starter Drew Pomeranz and that both could return soon, Thornburg perhaps next week. The Yankees, meanwhile, think Masahiro Tanaka (hamstrings) will be back in the rotation next week.

Unlike last year with Rafael Devers, the Red Sox do not have an elite prospect ready to help the big club. They signed veteran Brandon Phillips to a just-in-case minor league deal this week. He has a July 15 opt-out. They also have Adam Lind (who twice signed minor league deals with the Yankees before exiting) hitting well at Triple-A. He has a Sunday opt-out, but could decide to stay as injury insurance.

Mostly, though, Dombrowski and Cashman are on the clock between now and July 31 at 4 p.m. to, in Cashman’s word, “import.” That is the non-waiver trade deadline, and as Dombrowski noted: “You control your own destiny until July 31, afterward with waivers you don’t unilaterally control your own destiny. You acknowledge [when you have a contender] that it is your responsibility to do all you can [by July 31].”

Boston actually has less of an obvious to-do list, perhaps a deepening of the bullpen. The Red Sox have, by far, the majors’ largest payroll and hover near the $237 million luxury tax threshold that triggers particularly harsh financial and draft penalties. Dombrowski said, “Ideally we won’t go above $237 million,” but added, “there is no mandate from above” to stay below that total.

Cashman is following Hal Steinbrenner’s dictate to stay below the $197 million luxury tax threshold — the lowest level. That should not keep the Yankees from meeting Cashman’s checklist of “pitching, pitching and pitching.” Ideally, the Yankees want a difference-making starter, but the best available just might be Toronto’s J.A. Happ (who is actually pretty good). Ideally, they would add one more starter. Cashman said he is not prioritizing handedness, but a lefty would fit best.

He has a month to pull that off, though even after July 31, tinkering and more continues. Again, the roster hardly stays static for long, especially when the prize is Division Series versus wild card, notably when the rivalry is this intense and the Yankees and Red Sox are trying to outdo each other — on the field and off.