Regardless of how much stronger the center position is than the common narrative on its weakness would have you believe, productive fives are still the most valued commodity out there. And when you've got three when you only need two, you deal from the position of strength to concurrently fill one of your weaknesses. This is what the Nuggets intended to do this summer when they traded Koufos to the Grizzlies, receiving in exchange backup power forward Darrell Arthur and the rights to draft-and-stash big man Joffrey Lauvergne.
In theory, the Nuggets dealt an excess asset to fit a more pressing weakness. However, both of those criteria are subject to scrutiny. Importantly, the word "backup" in that description of Arthur is doing rather a lot here. Arthur is a decent player, indisputably, but he is also an average to decent backup at a position where that is not too hard to come by. Arthur helps a team on both ends of the court, but not hugely - he is a solid placeholder until the star player returns after the timeout, and not capable of much more than that.
The same is somewhat true of Koufos, a quality player, but a fringe starter in light of the aforementioned league-wide depth at the center position, and certainly only a backup in Memphis behind the reigning Defensive Player of the Year, Marc Gasol. But if you're trading a backup for a backup, you should be sure you're getting the better backup. Put simply, Koufos is better than Arthur, by a noticeable amount. This is true of both his value as a player and his value as an asset. Not only has Arthur yet to produce any season of the quality that Koufos did last year (8.0 ppg, 6.9 rpg, 1.4 bpg, 17.2 PER), but he is also one year older and with a much greater injury history. By no performance metric did Denver upgrade in this swap, and Lauvergne does not offset it. And ultimately, rather than balance their roster, they only further unbalanced it.
You can sign a player of Arthur's calibre much more easily than one of Koufos'. This is a market in which, for example, Tyler Hansbrough signed for one guaranteed year at a shade over $3 million, a highly comparable player on a highly comparable contract available without needing to lose a starting center. Indeed, Denver knows the power forward market well after having themselves made a splash in it this summer - despite trading for Arthur, they subsequently threw three years and $16.15 million at J.J. Hickson. If they needed a power forward, free agency could have addressed it. Alternatively, the Nuggets could have not looked outside at all, and instead relied upon the enigmatic but hugely talented Anthony Randolph to fill the small backup power forward hole - unreliable as he may be, Randolph needs the opportunity to succeed, or he never will.
The point here is that, even if starter Kenneth Faried does indeed go on to be traded, Koufos did not need to be. In light of the stress fracture McGee has just suffered that rules him out indefinitely, the once great center depth is now a position of weakness for the Nuggets, with Mozgov being the only healthy player at the position. Hickson might go some way to fill this void, but Koufos definitely would, yet he was moved for a player whose role could have and should have been filled by the player they subsequently signed or the player they already had. As third choice power forward, Arthur ranks 12th on the team in minutes per game - when a fringe starting center under the age of 25 tied down to a tiny $3 million contract is traded, it is imperative that he returns much more than that.