The Yankees' offseason roared to life Friday, when they signed elite reliever Andrew Miller and engineered a three-team trade for shortstop Didi Gregorius.

The splashy moves fill two critical needs for the Yankees, who fortified their bullpen by signing Miller to a four-year, $36-million contract. The signing came shortly after the Yankees, Tigers and Diamondbacks agreed to a deal involving Gregorius, who has long been targeted as an option to fill the void left by Derek Jeter's retirement.

"We are not a finished product,'' said general manager Brian Cashman, who might just be getting started.

The Yankees had appeared passive in comparison with AL East rivals such as the Red Sox and Blue Jays, who have infused their rosters with talent. But with the winter meetings approaching, Cashman described himself as being in "acquisition mode.''

Cashman did not rule out bringing back incumbent closer David Robertson, who reportedly has angled for a four-year deal worth north of $50 million. By all indications, the demand is too rich for the Yankees, who might move toward bolstering a rotation that is rife with injury risks.

The Yankees cut into their pitching depth to trade for Gregorius. They sent righthander Shane Greene to the Tigers, who in turn sent pitcher Robbie Ray and infield prospect Domingo Leyba to the Diamondbacks. The Diamondbacks sent Gregorius to the Yankees, who had pushed for him more than once beginning last offseason.

Gregorius, 24, is likely to step into Jeter's old territory, but they possess disparate skill sets. For most of his future Hall of Fame tenure, Jeter gave the Yankees a rare above-average bat at shortstop that more than made up for his shortcomings on defense. In Gregorius, the Yankees have a player whose value stems primarily from what rival executives called outstanding defense -- at the expense of a bat that has yet to develop.

The lefthanded-hitting Gregorius batted .226 with six home runs in 80 games with the Diamondbacks, faring much better against righties (.247) than against lefties (.137). It's part of the reason Cashman said he will begin his Yankees tenure as part of a platoon, with light-hitting defensive whiz Brendan Ryan slated to face lefties.

Several rival executives and evaluators believe Gregorius still can improve, a sentiment echoed by Cashman during a conference call with reporters.

"We think there's more in the tank there as he continues to develop,'' he said.

The addition of Miller, 29, gives Joe Girardi a formidable weapon to pair with breakout star Dellin Betances. Miller posted a 2.02 ERA and racked up 103 strikeouts in 621/3 innings last season for the Red Sox and Orioles. His deal eclipses the previous mark for setup men, held by Rafael Soriano, whom the Yankees signed to a $35-million contract before the 2011 season.

The Yankees emerged as the winners for Miller's services, though multiple reports indicated that the Astros submitted the highest bid at four years and $40 million for the lefthander. Cashman acknowledged the higher bid, which he said motivated the Yankees to move their offer up from $32 million.

But Cashman remained noncommital about the complexion of the bullpen. Without Robertson, the Yankees lack an experienced closer, and Cashman insisted he did not discuss that role with Miller.

The Yankees could add a closer, perhaps even Robertson, or remain with the group already assembled.

"I'm not opposed to continuing to pursue someone that has obviously known closing abilities versus the 'maybe he can do that,' '' Cashman said. "We may very well have the saves necessary sitting right here on our roster currently. But I'm not ready to see if that's the case or rubber- stamp that on this date that's what we're going to do. We're not in that mode at this time.''