Need an impact arm or bat that can give your fantasy team a boost? Whether you're looking for an injury replacement or someone to stash on your bench, the fantasy waiver wire has a plug or long-term fix to pick up today. Check out the following 10 players -- all sitting on waivers in many leagues, and all ready to help your team in Yahoo Sports Fantasy Baseball, the Official Commissioner Game of MLB. All statistics and ownership percentages are as of Wednesday.
Starlin Castro, second baseman, Marlins (48 percent owned)
Castro has been quietly hot with the Marlins, opening the season with an eight-game hitting streak and batting .290/.346/.319 with a 7:14 BB/K ratio. Only two of his 20 hits have gone for extra bases thus far, although a drop in power was to be expected following the 28-year-old's move from Yankee Stadium to Marlins Park. The .282 career hitter has batted third in all of the Marlins' games thus far, so he should be able to provide useful counting stats alongside his strong batting average.

Jose Pirela, outfielder, Padres (47 percent owned)
Pirela broke out in his first season as an everyday player last year, slashing .288/.347/.490 with 10 home runs and 25 doubles in just 83 games. The 28-year-old has carried over that success into 2018, batting .289/.322/.422 with 24 hits -- tops in the National League through Tuesday. The impressive power he displayed last season hasn't quite materialized yet, although he does have nine extra-base hits and a pair of steals as well. Now that he's batting either first or second most games, he's worth considering in five-outfielder leagues.
Yuli Gurriel, first baseman, Astros (39 percent owned)
Gurriel returned from his season-opening suspension on April 13 and immediately reclaimed the Astros' starting first-base job. Now that he's back in the fold, the 33-year-old projects to be a productive bat in Houston's potent lineup after slashing .299/.332/.486 with 18 home runs, 75 RBIs and 43 doubles as a rookie last year. Gurriel appears capable of hitting for a high average again after striking out in just 11 percent of his plate appearances last year, and he still has room to grow if he improves on his career 3.9 percent walk rate.

Trevor Williams, starter, Pirates (39 percent owned)
Williams, who went 7-9 with a 4.07 ERA and a 1.31 WHIP as a rookie last year, has gotten off to a great start this season with three wins and three quality starts in his first four outings. Even in his lone loss he pitched well, yielding just two earned runs in six innings. While the 25-year-old's 1.93 ERA will inevitably come up, his 3.69 FIP and 1.20 WHIP suggest he might build on his rookie success.
Christian Villanueva, third baseman, Padres (38 percent owned)
After launching three home runs against the Rockies on April 3, Villanueva proved his outburst wasn't a fluke by homering in three straight games from April 14-16. He showed impressive power as a September callup last year as well, slugging four home runs in just 12 games. His 33 percent strikeout rate will cause his .327 average to come down, but there's serious power potential here if the 26-year-old continues hitting in the middle of San Diego's lineup on a regular basis.

Bud Norris, reliever, Cardinals (37 percent owned)
Norris has cemented himself as manager Mike Matheny's primary closer by converting all four of his save opportunities, including three during a four-game span from April 13-17. The former starter, who notched 19 saves with the Angels last year, has been lights-out with a 1.93 ERA and a 17:1 K/BB ratio in 9 1/3 innings. While he may return to a setup role when Greg Holland joins the team, it's possible he'll continue handling ninth-inning duties given how well he's pitched. For now, he's a must-add in all formats.
Chris Devenski, reliever, Astros (36 percent owned)
The versatile swingman seems to have supplanted Ken Giles as the Astros' closer after receiving (and converting) both of Houston's save opportunities within the last week. Even if Devenski is still sharing closing duties, he remains a valuable fantasy asset due to his ability to provide strong ratios and strikeout totals over a large quantity of innings. In 2016-17 combined, he went 12-9 with a 2.38 ERA, a 0.93 WHIP and a 9.7 K/9 rate across 189 innings. After posting an 11.2 K/9 rate last year, he has the swing-and-miss stuff to excel in the ninth inning and needs to be owned in all fantasy leagues.

Kole Calhoun, outfielder, Angels (27 percent owned)
Calhoun hasn't recorded an extra-base hit since going 3-for-5 with a triple and a home run on Opening Day, but he's started picking it up at the plate by hitting safely in seven of his last nine games while notching six RBIs. He appears to be pressing based on his 2:18 BB/K ratio, which looks like an anomaly compared to his 138:252 BB/K ratio in 2016-17 combined. The 30-year-old is posting the highest hard-hit rate of his career at 45.1 percent, so he should still be considered a viable power source after averaging 20 homers and 28 doubles from 2014-17.
Mallex Smith, outfielder, Rays (24% owned)
With Kevin Kiermaier (right thumb) landing on the 60-day disabled list this week, Smith has taken over as Tampa Bay's everyday center fielder. Smith is off to a hot start at the plate, slashing .383/.431/.511 from primarily the bottom of the order, so he could see a boost in value if he moves up in the lineup. While the 24-year-old has yet to show much power at the Major League level with just five home runs in 168 career games, he has the speed to hit for average and make an impact on the bases. After tallying 32 steals across 153 games in 2016-17, Smith has the potential to top 30 in a full season.

Joe Mauer, first baseman, Twins (20% owned)
Just when Mauer appeared to be entering the decline phase of his career, he bounced back to hit .305/.384/.417 with a 36.4 percent hard-hit rate last year -- his highest mark in each category since 2013. Mauer's continued hitting well into '18 as his hard-hit rate has held steady, and he's demonstrated great patience at the plate with a 12:7 BB/K ratio through 11 games. The 34-year-old doesn't offer much pop for a first baseman, but his elite contact skills and batting eye still make him a fantasy boon in deeper formats.