Derrek Lee of the Chicago Cubs hasn't been able to maintain the offensive standard he's established over the course of several seasons. ((John Smierciak/Associated Press))

At this point during last year's baseball season, perennial all-stars like Vladimir Guerrero, C.C.Sabathia and David Ortiz were playing a mediocre brand of baseball. One year later, the three superstars are back on track and lit it up at the 81st annual midsummer classic (and home-run derby). 

In fantasy baseball terms, the rebirth of big stars may have allowed for a few late-round steals on draft day, like Guerrero, who went in the 15th round in Yahoo! fantasy baseball leagues on average. But like any year, there are enough other big leaguers in major slumps to fill the void and keep fantasy owners pulling their hair out.

Here's a look at the biggest flops through the first half of the season:  

Zack Greinke (KC - SP)

For a guy that was selected in either the second or third round of Yahoo! fantasy league drafts this season, Zack Greinke hasn’t pulled his weight. Last season, the Royals’ ace set career-highs in wins (16), complete games (six), shutouts (three), strikeouts (242) and ERA (2.16).

Greinke is just 5-8 with a 3.71 ERA this season. What’s more disturbing, though, is that he is about to surpass or has already surpassed last year’s totals in runs, earned runs and home runs allowed.

Trevor Hoffman (MIL - RP)

Last season, not many people thought Trevor Hoffman could still be the unhittable closer of years past. He proved them wrong by posting 37 saves (out of 41 opportunities) with a stellar 1.83 ERA for Milwaukee.

Perhaps the naysayers were just one year too hasty.

At age 42, Hoffman has all but lost his fantasy value. With a 2-4 record to go along with just five saves and a career-worst 8.33 ERA, we just may be witnessing the end of a Hall of Fame career.

Derrek Lee (CHI - 1B)

Let’s put it this way, Derrek Lee’s average has been between .286 and .335 for the past five seasons. He’s been a home run and RBI machine over the past several years with the Cubs and, with the exception of an injury-plagued season in 2006, he’s been a model of consistency.

That’s what makes his struggles this year so troubling. At age 35, Lee is batting just .239 with 38 RBIs. What’s worse is that with 82 strikeouts at mid-season, he’s on pace to match or beat his previous high of 164 set back in 2002.

Yunel Escobar (TOR - SS)

There’s a reason why Atlanta wasn't willing to part with Yunel Escobar before, but he’s now the newest member of the Toronto Blue Jays — he’s been offensively challenged.

Though Escobar is considered one of the better defensive shortstops in the game, his .238 average and whopping zero home runs are a tough pill to swallow in fantasy baseball terms – even if he is a shortstop.

To put it in perspective, Escobar was drafted in 100 per cent of Yahoo! fantasy leagues, but is now owned in just 45 per cent of them.

Carlos Zambrano (CHI - SP)

It’s hard to explain what's happened to Carlos Zambrano.

Most avid baseball fans know the Cubs ace as a visibly emotional pitcher who won an average of 15 games, while racking up nearly 200 strikeouts between 2003-08.

But that hasn't been the case this year. In his first start, he surrendered eight earned runs, while lasting just 1.1innings.

It was a harbinger of things to come. Zambrano was moved to the bullpen and ultimately lashed out at teammate Derrek Lee on June 25 in the Cubs dugout. This week marks the first time since then that the Venezuelan has thrown at all, as he’s been in New York completing an anger management course.  

Jason Bartlett (TB - SS)

Considering that Rays shortstop Jason Bartlett set career-highs in every single offensive category in 2009, his stats up to this point one year later are disappointing, to say the least.

The most glaring numbers are his home runs, stolen bases and batting average. He had 14 HRs with 30 steals and a .320 average last year, but just two HRs, four steals and a .231 average this year.  

Rick Porcello (DET - SP)

It might be hard to believe now, but at the midway point of last season, Rick Porcello was a realistic candidate for American League rookie of the year. He had five straight wins in May and was making a name for himself in Detroit.

It seems the only people who know his name now are those that had the misfortune of drafting him this year. Oh, and his teammates in the minors. The Tigers' right-hander has a record of 4-7 with a 6.14 ERA and hasn't seen the light of day in the majors since a bad loss against Arizona on June 19.  

Gordon Beckham (CHW - 2B/3B)

Like Porcello, Gordon Beckham turned many heads when he landed so comfortably in the majors last season. He had 41 hits in 135 at bats for a .304 average. As an everyday player in 2010, the White Sox infielder is just 61 of 276 for a measly .221 average, though he has shown some promise of late.