The Giants are most likely to start a two-time Pro Bowler and three-time 1,000-yard rusher still in his prime at running back in Week 1 against the Cowboys.

His name is Saquon Barkley.

Because Barkley is not expected at training camp after he and the Giants failed to bridge a small gap on a multiyear extension by Monday’s deadline, fans shifted focus to second-stringer Matt Breida, rookie Eric Gray and big-name free agents Dalvin Cook and Ezekiel Elliott as possible replacements.

But the truth is that Barkley still is the most likely season-opening starter, and there are some negotiation tactics left that could accelerate an unhappy reunion.

Barkley, 26, is forced to either play on the yet-to-be-signed one-year franchise tag or sit out games, and he is prohibited from negotiating a long-term extension until after the season.

By skipping training camp — he is not subject to fines and lost wages do not begin until Week 1 (about $560,000 per week) — he reduces his injury risk and sends a message of displeasure while forcing the Giants to look at their weakened offense.

But there are provisions that could be added to the tag, which is not as iron clad as it sounds.

If the Giants get antsy about Barkley’s absence in the preseason spilling into the regular season, they could offer a clause forfeiting the right to use a second franchise tag next offseason — assuring Barkley of free agency in 2024 — in exchange for him signing the $10.1 million tag and reporting sooner.

“At that position, be careful what you ask for,” CBSSports.com contracts analyst Joel Corry told The Post. “That puts a lot of pressure on this year.

“But if they don’t want to tag him next year then it’s a meaningless gift they could give.”

Another incentive the Giants theoretically could offer to expedite Barkley’s arrival is a slight raise over the tag-guaranteed baseline salary, though Corry could not think of an instance when that happened.

“I’d say 99.5 percent of the time you are going to play for your tag amount,” Corry said, “but the one-year salary is negotiable.”

Edge rushers Jadeveon Clowney and Yannick Ngakoue both previously agreed to play for less than the tag to facilitate trades.

Barkley has to sign the tag to be traded, but it is not thought that he wants to be traded nor that the Giants are interested in trading their best playmaker before a season of big expectations.

The Giants’ final three-year offer with an average annual value between $11 million and $11.5 million per year and $22 million and $23 million guaranteed was less than $2 million away from Barkley’s final asking price, sources told The Post.

“The Giants should’ve done it if they were offering $13 million [per year] in March because that’s a win right there,” said Corry, a former NFL agent. “You were never going to get him for [less than] the guarantees of tagging him twice ($22.2 million). It’s a misstep any time you are trying to win the deal rather than finding something both sides can live with.

“Saquon would’ve been more uncomfortable than the Giants if he got what he wanted [at the end], anyway.”

If the bad feelings linger into the regular season, here are other options (four in-house) for the Giants’ Week 1 running back:

Thirteen of Breida’s 20 career starts were for the 2018 49ers.

He has just 175 offensive touches over the past three seasons combined — Barkley had 352 last season — so there is a question of whether he still can handle the heavy workload.

The rookie is a three-down back in position coach Jeff Nixon’s eyes.

He is the biggest beneficiary of Barkley’s missed time as the Giants can test the popular theory that most late-round and undrafted running backs with fresh young legs can be productive behind the right offensive line.

He averaged 6.4 yards per carry and scored 11 touchdowns from scrimmage at Oklahoma last season.

Primarily a special-teamer, Brightwell has 32 carries through his first two seasons.

He scored a key touchdown when Barkley was sidelined for a drive against the Packers.

An undrafted rookie last season, Corbin did not play a snap in his one game elevated from the practice squad.

There is no indication that the Giants ruthlessly would rescind Barkley’s tag — adding $10.1 million to their $4.1 million in salary cap space — to pursue a free-agent veteran with a comparable résumé and save a few bucks at the risk of angering the locker room.

Cook reportedly already has an offer from the Dolphins and is looking for “a significant offer” that could result from a bidding war.

Elliott seems to have fewer options, but his 1,881 career carries — compared to Barkley’s 954 and Cook’s 1,282 — and career-low 3.8 yards per carry last season suggest he might be running on fumes.

It is more likely the Giants sign a lesser free-agent back after a group veteran tryout during camp.