The Cavaliers’ LeBron James turned 31 on December 30th. (AP Photo/Marcio Jose Sanchez)

As is often said of aging athletes, Father Time is undefeated. And as great as Western Conference leaders Golden State and San Antonio have been to start the season, the signs of possible athletic decline from LeBron James might be the biggest obstacle to the Cleveland Cavaliers finally winning an NBA title.

James turned 31 on December 30th. Assuming he stays relatively healthy, James will finish the regular season in or near the top 40 in all-time minutes played (he’s currently 50th according to and is already seventh in career postseason minutes. As superhuman as he has appeared throughout his career, it would be more surprising if there weren’t some signs of decline.

Certainly, the last few years have seen a noticeable downturn in James game-to-game defensive impact from his peak during his two championship seasons with Miami. Some of this is simply energy conservation from a player who has crammed more than a full season’s worth of playoff games (107) into his five consecutive Finals appearances. But the effect has been fairly clear in box score-based stats as 2014-15 saw James post career low Defensive Win Score along with his second lowest Defensive Box Plus Minus. (It should be noted that though the change from Miami to Cleveland makes direct comparison of on/off plus-minus metrics more difficult, James has performed well in ESPN’s Defensive Real Plus/Minus in both 2014-15 and 2015-16 after an indifferent 2013-14).

It’s assumed that James would dial the defensive intensity back up in the crucible of the playoffs this season. The 2015 postseason seemed to support this, as Cleveland’s defense went from slightly below average to well above average in per possession efficiency. But what if the decline is just that: a genuine diminution in physical capability?

In particular, James’s ability to overwhelm opponents at the basket with his combination of speed and strength could be on the wane. Looking at NBA play-by-play logs over the course of his career, James had his shot blocked at the rim (shot from five feet or fewer) between 5 and 6 percent of the time every year from 2008-09 through his final year in Miami in 2013-14. Since then, this number has skyrocketed, to 8.2 percent in 2014-15, and 9.4 through Monday’s win over Toronto. Though this is still below the league average of 11.7 percent of close shots blocked this season, this sharp increase could signal a decline in James’s ability to go over, around and straight through contesting defenders.

Unsurprisingly, James efficiency on those close shots has plummeted, but perhaps more concerning is his decline on those shots which haven’t been blocked.

If LeBron is fully exiting the prime of his career, it has potentially severe implications for the Cavaliers’ championship aspiration both this season and beyond. Cleveland has expended the bulk of its tradable and free agent assets already. The roster on hand (and the attendant enormous luxury tax bill) is built on James continued dominance, especially on offense. As that dominance wanes, the margin for error or injury to other key pieces such as Irving or Kevin Love diminishes to near nothing.