CINCINNATI -- One of the most aggressive teams on the free-agent market all winter, the Reds made another big addition on Monday. The club signed outfielder Nick Castellanos to a four-year, $64 million contract with a $20 million mutual option for the 2024 season.

Castellanos will earn $16 million in 2020, $14 million in '21, $16 million in '22 and $16 million in '23. The option for '24 has a $2 million buyout. He also has opt-out clauses following the '20 and '21 seasons, by which he can become a free agent.

Cincinnati could now become the favorite for the National League Central after a six-year postseason drought.

"Nick was one of the most attractive options on the market. He has been a consistently elite producer offensively and that complements our existing team well," Reds president of baseball operations Dick Williams said in a statement.

To make room on the 40-man roster, outfielder Jos¨¦ Siri was designated for assignment. Siri was the organization's No. 15 prospect, according to MLB Pipeline.

Improving their offense after a disappointing 75-win season was the Reds' top priority going into the offseason. The team was 12th in the NL in runs scored and batting average and led the Major Leagues in one-run losses.

In response, the Reds signed second baseman Mike Moustakas (four years, $64 million) and outfielder Shogo Akiyama (three years, $21 million). For good measure, Cincinnati also upgraded its rotation with December's signing of lefty Wade Miley to a two-year, $15 million contract.

All of the new contracts add up to $164 million allocated to the four free agent acquisitions.

Castellanos, who will turn 28 on March 4, batted .289/.337/.525 with 27 homers and led the big leagues with 58 doubles over 151 games combined for the Tigers and Cubs last season. But his season really took off in Chicago after a July 31 trade from Detroit as the right-handed hitter notched 16 homers with a 1.002 OPS in 51 games.

Over seven Major League seasons and 888 games, Castellanos is a career .277/.326/.471 hitter with 120 homers. His 229 doubles since 2014 ties him with Boston's Mookie Betts for most in the Majors in that time.

After Akiyama was signed, Williams and general manager Nick Krall indicated that the Reds were not finished making moves. They were still considered in the hunt to get either of the top free-agent outfielders -- Castellanos or Marcell Ozuna. The Braves agreed to a one-year, $18 million contract with Ozuna last week.

The biggest question looming over the Reds is what becomes of their current stable of outfielders. With Castellanos and Akiyama expected to play regularly, there is only one starting spot open for center fielder Nick Senzel and corner outfielders Jesse Winker, Aristides Aquino, Phillip Ervin and Scott Schebler. Both Senzel and Winker could be valuable trade assets for Cincinnati, which previously identified the shortstop position -- currently held by Freddy Galvis -- as one area that could be upgraded. The team is also interested in bullpen help but has also signed relievers Tyler Thornburg and Nate Jones to Minor League deals over the past month.

No matter how the latest move shakes up the roster, the Reds have positioned themselves as a team to watch as it hopes to contend in 2020.