PHILADELPHIA -- How much can one player improve a defense?
Perhaps quite a bit. The Phillies acquired shortstop Jean Segura from the Mariners on Monday. He is not considered a top-tier defender, but he should be better than the foursome the Phillies played at shortstop last season. The Phillies' inclusion of first baseman Carlos Santana in the trade also allows them to move Rhys Hoskins from left field to first. The deal essentially upgrades the Phillies at two positions -- shortstop and left field -- which should help what was baseball's worst defense last season.
"I do expect our team defense to improve next year," Phillies general manager Matt Klentak said Monday evening. "I don't think you go from the bottom of the pack to the top of the pack in one offseason, but I do think you have to make every effort to improve wherever you can. This trade not only adds Segura to solidify the middle infield for us, but it also allows Rhys to slide to first base and be replaced in essence by an outfielder who's more comfortable out there, who has spent more time in left field. I think this really helps us in a couple of different spots. Some combination of external additions or position changes coupled with the improvement of our own players should lead to an improved team defense next year."

Here is a look at some of those positions and other possibilities for improvement:
Shortstop
Segura finished last season with a career-best five Defensive Runs Saved, according to FanGraphs. His DRS total has fluctuated over his career, but overall he is steady. In seasons in which he played more than 1,000 innings at shortstop he finished in the positive in 2013 (three), '14 (two) and '18 (five). He finished in the negative in '15 (-3) and '17 (-3).
He posted a zero DRS at second base in 2016.
But compare Segura's career at shortstop to the four Phillies that played there last season: Scott Kingery (-6), J.P. Crawford (-6), Pedro Florimon (-6) and Asdrubal Cabrera (-5).
If those numbers hold true, the Phillies should be better at shortstop in 2019. Ground-ball pitchers like Aaron Nola and Jacob Arrieta should love that.

Left field
Hoskins finished at -19 Outs Above Average, according to Statcast™. It was the second-lowest mark out of 259 outfielders in baseball. If the Phillies put somebody in left that can play average defense, it will be an improvement.
Even Nick Williams, who finished at -7 OAA, would be a slight upgrade.
But imagine for a moment the Phillies sign somebody like free-agent center fielder A.J. Pollock, who ranked 24th at six OAA. Theoretically, the Phillies could have some combination of Roman Quinn (four OAA), Odubel Herrera (zero), Aaron Altherr (three) and Williams on the corners and Pollock in center.
Suddenly, the Phillies would have a formidable outfield defense.
It should be noted that free agent Bryce Harper had -12 OAA last season, although his agent, Scott Boras, told MLB Network insider Ken Rosenthal recently that Harper's defensive struggles can be pinned to a hyperextended left knee that kept him sidelined late in 2017. Boras expects better defense from Harper in 2019.
First base
Hoskins is not a better defensive first baseman than Santana. Santana finished at zero DRS last season and 10 in 2017. Hoskins has a much smaller body of work in the big leagues at first base, but he posted a combined -2 DRS in 310 innings over two seasons.
Still, there is no question first base is a better place for Hoskins and the Phillies than left field.
Third base
The Phillies still want to sign Manny Machado. It is unknown if he would play third base because his preference is shortstop. But if the Phillies crush the competition in terms of a contract offer, he could change his mind.
Machado is a better third baseman than he is a shortstop.
Since 2012, only Nolan Arenado (109) has more DRS at third base than Machado (84). Machado also has the best UZR (50.6) among third basemen in that time. Compare those numbers to Phillies third baseman Maikel Franco last season: -12 DRS and -5.0 UZR.
Second base
Cesar Hernandez posted a -12 DRS last season and a -2 DRS in 2017. He had four DRS in '16.
Maybe Larry Bowa could work with Hernandez in Spring Training. He is an excellent teacher. The Phillies also have an internal alternative if Hernandez does not improve. Kingery is regarded as a brilliant second baseman, although he played only 23 innings there last season. If the Phillies trade Hernandez before Spring Training, Kingery is expected to take his place.