Buster Posey got his shortstop. He didn’t get his ace.

Right-hander Corbin Burnes agreed on a six-year, $210 million contract with the Arizona Diamondbacks on Friday night, the largest contract in franchise history. Now, if the Giants are to upgrade their starting rotation this offseason, they will have to fall back on their contingency plans.

San Francisco had reportedly been a favorite to sign Burnes, who grew up in Bakersfield and attended Saint Mary’s College in Moraga. Burnes, winner of the 2021 NL Cy Young Award, joins right-handers Zac Gallen and Merrill Kelly on the Diamondbacks, a divisional rival that scored more runs than anyone else in the majors last season.

Arizona wasn’t the only NL West team to get stronger on Friday. Outfielder Teoscar Hernández agreed to return to the Los Angeles Dodgers on a three-year, $66 million deal. Along with Hernández, the defending World Series champions have signed a pair of former Giants in left-hander Blake Snell and outfielder Michael Conforto as well as right-handed reliever Blake Treinen and utility man Tommy Edman (extension). The Dodgers, unsurprisingly, are also a favorite to land Japanese right-hander Roki Sasaki.

The San Diego Padres have yet to make a true impact move this offseason, and there’s a realistic chance they lose shortstop Ha-Seong Kim and outfielder Jurickson Profar in free agency. Still, the Padres won 93 games and nearly upset the Dodgers in the NLDS, taking a 2-1 series lead before their offense faltered. They, like the Dodgers, are a favorite to land Sasaki.

Signing Burnes would have unquestionably given the Giants a better chance of returning to the playoffs, and several of San Francisco’s players were vocal about the team’s pursuit of Burnes.

After signing a franchise-record seven-year, $182 million contract with the Giants, Willy Adames said at his introductory news conference that he would be in contact with Burnes, who he played alongside with the Milwaukee Brewers. Ace Logan Webb called Burnes “almost a perfect match” during an interview with Chris Rose. Now Adames and Webb will have to spend much of the next half-decade competing against Burnes.

San Francisco could head into the season with a rotation of Webb, Robbie Ray, Jordan Hicks, Kyle Harrison and Hayden Birdsong — the same quintet they had in place entering the Winter Meetings.

The best available unsigned starting pitcher is right-hander Jack Flaherty, who bounced back last season (3.17 ERA, 162 innings) and won a World Series with the Dodgers after struggling from 2020-23 (4.42 ERA, 299 innings). Aside from Flaherty, there aren’t many true impact arms on the market.

In addition to Flaherty, San Francisco’s options include Justin Verlander, Max Scherzer, Charlie Morton, José Quintana, Patrick Corbin and Kyle Gibson, among others.

San Francisco will also likely make a pitch to Sasaki, but they’ll have to beat out the Dodgers or Padres.

The Giants still need to upgrade their starting rotation if they want to compete in a stacked NL West next season. How they do so at this juncture is unclear.