The 17th pick in the 2012 draft, Zeller was handed the starting center spot last season after Anderson Varejao split a muscle in his leg. He averaged 7.9 points and 5.7 rebounds in 26.4 minutes, but shot just 43.8 percent and lagged on the defensive end.
This season, while so much has gone wrong in Cleveland, Zeller has been one of the lone bright spots.
Buried for a while behind Andrew Bynum and Anderson Varejao to begin the season, Zeller has since seen a larger role thanks to the trade of Bynum. We've seen a remarkable improvement already, with even bigger opportunities possibly ahead.
Here's how Zeller has progressed from his rookie season, and why his development is so important for the Cavs.
One of the biggest knocks on Zeller last year was his toughness, or lack thereof.
Zeller was listed at 7-feet and 247 pounds coming out of UNC. According to the Cleveland Plain Dealer, he played most of the year around 244 pounds, but dropped to 238 near the end of the season due to the flu. Judging by the way Zeller was being pushed around on a nightly basis, 238 may have been a generous amount.
Now, he's far from the first rookie center to struggle. Even Roy Hibbert of the Indiana Pacers went through early growing pains when he first entered the league. Big men always take longer to develop than guards, and players like Zeller and Hibbert are perfect examples of that.
Zeller's frame was definitely an issue and prevented him from becoming a physical defender and low-post scorer. This offseason, Zeller employed an interesting strategy of feasting on Krispy Kreme doughnuts, one which the rest of us non-professional athlete types would love to try.
With the doughnuts came some significant weight gain, about 20 pounds in total, which pushed Zeller to his heaviest frame yet. As he described in the same Cleveland Plain Dealer article:
"I'm not going to say I was fat, but I felt very out of shape,'' Zeller said. "I had to transfer that into 'good' weight and muscle. Now I'm between 255-260, and I'm pretty happy with that, although I'm trying to gain more.''
An emergency appendectomy in the preseason served as a setback for Zeller's weight gain, although he's now back up to around 250 pounds.
The increase in muscle definitely shows.
Last season he spent the majority of his time on offense outside the paint, preferring jumpers to banging bodies with opposing centers. According to 82games.com, Zeller took jump shots 58 percent of the time. While it's nice to have a 7-footer who's capable of doing such a thing, Zeller was almost abandoning his post game.
This year Zeller has been much more aggressive inside, has reduced his jump shots to just 43 percent of his offense and has increased both his inside scoring and dunk numbers.
An improved physique has allowed Zeller to still hit the 18-footer while increasing his time spent in the paint.
As previously mentioned, Zeller shied away from the paint and the contact that came with it quite frequently.
In fact, 48 percent of his total shots were from the mid-range area. The problem with this was, Zeller only made 36.6 percent of them.
Zeller's 43.8 percent shooting from the field was tied for 28th among qualified centers last season. Too many low-percentage shots, including these mid-range jumpers, doomed Zeller's offensive efficiency.
This season Zeller has spent more time in the paint, which has drastically increased his shooting percentages. Now up to 54.4 percent from the field, this number would rank sixth among all centers if he had enough games to qualify. Because opponents now have to respect his post game, Zeller is in turn getting more open looks from the mid-range area.
Speaking of this area, Zeller has been much more selective in his looks. He takes just 22 percent of all his shots from mid-range, down from 48 percent a year ago. He's also getting better looks due to the respect factor. Zeller is now converting 57.7 percent of his jumpers from this range, up from the 36.6 percent of a year ago.
He's beginning to form nice pick-and-roll chemistry with Kyrie Irving and Dion Waiters, which is also helping lead to these open jumpers. Teams have to pay so much respect to Irving and Waiters in these situations that Zeller is getting more uncontested looks.
Given that starting power forward Tristan Thompson isn't a good jump-shooter, it's nice to have an option like Zeller at center next to him.
No one would go as far to call Zeller a great defender, but he's improving.
Strength was an issue for Zeller last season, as was quickness. The Cavs run a lot of defensive sets requiring the center to slide out and cut off the opposing point guard on pick-and-roll sideline plays. It's then the center's responsibility to get back to his original man, or open post player, as quickly as possible.
Zeller can still be slow on these, and often gets into foul trouble because of it. His blocked shot/shooting foul rating was just .45 last season (via 82games.com). For comparison, Hibbert's is 1.40 this year.
Zeller has been quicker on his recovery this season, and is doing a better job of blocking shots without fouling. His blocked shot/shooting foul rating is up to .84 in 2013-14. Still a long way from a player like Hibbert, but nevertheless a nice improvement from his rookie year.
Here are some other key defensive stats and ratings showing Zeller's improvement from last season. All stats are on a per-36 minute basis.
Zeller's added muscle has made a big difference on the boards.
He's now pulling in nearly 20 percent of all available defensive rebounds, and is up to 9.7 total boards per 36 minutes.
Steals and blocks are slightly improved, and his defensive rating is down three points from a season ago.
Zeller's continued development is key to the Cavs' center position looking ahead to next year.
What is Zeller's ceiling as a pro?
Anderson Varejao, while still producing at a high level, has just a partially-guaranteed contract for 2014-15. Cleveland can cut him and shed nearly $10 million from their salary cap. For someone who's had such bad luck with injuries lately, keeping Varejao on the roster is far from a sure thing.
Zeller needs to be ready.
The future of the Cavaliers is very uncertain right now. After a change of general manager, we still don't know if Cleveland will be buyers or sellers at this year's trade deadline.
The Cavs could even choose to move Varejao before the deadline, leaving Zeller as the starting center once again.
Regardless of what happens, Zeller needs to keep improving and show that he can handle a starting job.
Something has to go right for the Cavs' this year, and Zeller playing at a high level may be just that.
All stats via basketball-reference.com unless otherwise noted.