Rookies are always tricky to evaluate because they can look great one week and then completely fall apart the next. With some big name call-ups sticking around, what can we make of their initial success? More importantly, we need to figure out if these players are best used as keepers or trade bait.

Scouting players who may be undervalued and re-assessing players who may be overvalued is a weekly exercise that you should pursue. Fortunately, I have you covered. Here are a few players who are good buy or sell candidates based on their current performance compared to ownership levels. This could mean scooping them off waivers if possible or actively seeking a trade to acquire or discard certain players in order to maximize value.

I will include one player at each key position group (Infield, Outfield, Pitcher). Ownership levels are taken from Yahoo! Fantasy Sports.

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Underrated Players - Week 9

Ian Happ (2B/OF, CHC) - 42% owned

Happ was thought to only stick around for a couple of days while the Cubs sorted out injuries in their infield, but low and behold he's still here. He owns a slash line of .268/.375/.561 with two home runs and five RBI in 41 at-bats. The 33.3% K% is a bit concerning, but his minor league averages are a full 10 points lower so the strikeouts should reduce with time and experience. He has shown a good eye by walking 14.6% of the time.

Happ is in a great situation to drive in and score runs in the Chicago lineup, but the most encouraging thing is that he's been batting fourth or fifth nearly every game. We're not talking about a rookie who is filling a void in the eighth spot for a team lacking better options. Happ has a legit opportunity to stick in the most favorable of positions for fantasy opportunity. He needs to be owned in the majority of mixed leagues by now.

Gregory Polanco (OF, PIT) - 80% owned

Giving up on a young, budding star two months into the season isn't a wise thing to do. I've seen people selling low and even dropping Polanco in competitive leagues out of sheer frustration. It's downright shocking to think that someone who was a consensus top-20 outfielder worth a fifth or sixth round pick this year isn't even owned in 1/5 of leagues right now.

Polanco came off the 10-day DL in the minimum allotted time and then smacked a home run in his first game back on Friday. He's not a 40-HR talent, nor is he necessarily a 20/20 guy, but if he stays healthy the rest of the way, you should be looking at 20+ homers, 80+ RBI and double-digit steals easily. If someone in your league is willing to part ways cheaply, then strike now before he starts heating up.

Jose Berrios (SP, MIN) - 80% owned

How can a player who was the hottest waiver add of the last two weeks be underrated? Berrios went from a virtually non-existent ownership level (outside of a few deep keeper leagues) to being owned in as many leagues as Gregory Polanco. Three wins in three games and 22 strikeouts in 21 2/3 innings will do that. Berrios was a hot name last year around this time too, but it didn't go over nearly as well with an atrocious 8.04 ERA and 5.4 BB/9 over 14 starts.

This time it looks like he's figured things out and last year's line in the majors is a major outlier in his profile, as he traditionally walks less than the league average. This might be a good time to swing a deal for Berrios because he gave up three homers in his last start against Baltimore. Other owners might think to sell high before he invariably implodes, but that doesn't seem likely to happen.

Overrated Players - Week 9

Trevor Story (SS, COL) - 91% owned

This might come back to bite me, as it did last preseason when I predicted Trevor Story would be a spring training wonder only... Story came off the DL and homered in his second game back on Thursday, which was a huge relief for his disgruntled owners. The buy-low window on Story is closed now that he's back on the field, but in my opinion he isn't worth buying at all.

We know he's going to strike out a ton (32.2% K% with Colorado) but we're perfectly fine with that if he's mashing 30 home runs. The problem is that he's still hitting .190 on the year and must prove he can get up to speed with his averages to compete with the likes of Francisco Lindor, Carlos Correa, or Corey Seager. If I'm looking for an elite SS that I'd have to pay a pretty penny for in a money league, I'll take any of those three first.

Jose Bautista (OF, TOR) - 93% owned

Are we finally seeing the return of Joey Bats? Bautista went deep again on Saturday, giving him nine on the season. He's approaching the top-10 HR rank among outfielders, but his average still sits at .237. Bautista may have a good run coming, but when it's all said and done he will probably still finish with disappointing totals.

We can't ignore the fact that his strikeout rate has risen the last three season, up to 24.9% this year. If anything, he's actually bound for a dip in his .284 BABIP that's 17 points higher than his career mark, so the AVG could get worse. Finally, his 31.5% hard contact rate is also down, which is never a good sign for a slugger. If you already own him, you probably won't get enough return on value to flip him unless there's still someone out there that believes he will return to the glory days at age 36.

Masahiro Tanaka (SP, NYY) - 94% owned

I'm giving up. Tanaka should have been one of the safest, most predictable SP picks in 2017 drafts. Some reputable baseball analysts in the national media even made him their preseason Cy Young pick. It took one Opening Day start to squash those dreams, as he lasted just 2 2/3 innings, allowing two home runs against the not-so-vaunted Tampa Bay lineup. It simply hasn't gotten better ever since then.

Other than a complete game gem against the Red Sox, Tanaka has been getting pounded. Just in the last two weeks, he gave up six runs to Houston and then eight to his new nemesis Tampa. A 3.87 SIERA suggests Tanaka is pitching far better than his ratios look and his plate discipline stats don't look much different than the last couple of years. The problem that won't go away is the long ball. Tanaka has given up 13 HR this season and he plays in one of the most hitter-friendly parks, just three-hundredths of a point behind Chase Field in ballpark factor for home runs. For a pitcher who doesn't strike out a ton of batters, there may not be enough upside to chase if he is going to keep walking hitters at a higher rate and letting the ball leave the yard. Don't give up on him completely if you already own him, but don't go chasing him in the trade market either.

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