The Oklahoma City Thunder were, arguably, the league’s best team last year in the regular season. They did not have the best record, as Miami finished the year with 66 wins to OKC’s 60, but the Thunder played a much tougher strength of schedule and actually underachieved relative to their margin of victory (MOV) by about 4 games. As I mentioned in my post projecting Miami for next year, the Heat actually significantly overachieved relative to their expected wins based on their MOV and also faced the league’s easiest schedule. Unfortunately for the Thunder, in their first round series against the Rockets, they lost Russell Westbrook for the rest of the season with a torn meniscus suffered on this (dangerous) play by Patrick Beverly:
https://www.youtube.com/watch?v=LkYevpkhZYc
The Thunder managed to win that series without Westbrook, their second best player and a top 10 player in the league. They were stopped in the second round by the Grit & Grind Memphis Grizzlies. Kevin Durant simply had to do too much against one of the league’s very best defenses and, despite a heroic effort, he came up just a bit short.
This offseason, the Thunder lost Kevin Martin (+.33 xRAPM) to free agency and really did not replace him. They signed Ryan Gomes, who was not in the league last year and was hurt a lot and just not very good in 2011–12 the last time he played. They also drafted Steven Adams and Andre Roberson. RAPM projections really do not like Adams and I don’t have any sense of how good Roberson will be, but given his status as a second rounder, betting on him to help or play many minutes on a team contending for a title is a very iffy proposition. In fact, in my minutes projections, the two new Thunder rookies are not set to play at all.
One young player I do expect to play quite a few minutes, if only out of necessity, is Jeremy Lamb. The ostensible centerpiece of the James Harden trade, Lamb will need to take on some of the minutes covered by the departing Martin. He projects to improve over his very poor performance by xRAPM next year. He’s still sub-replacement level by these projections, but he needs to be developed and he shouldn’t be so bad that he can’t be out on the court. It’s likely returning guard Reggie Jackson will also see an increase in minutes as he backs up Russell Westbrook and plays alongside him in spots, again to mitigate some of the loss of Martin. Neither Lamb nor Jackson projects to be near the overall contributor that Martin was last year and as a result the Thunder will take a step back in overall quality. Last year the Thunder played like a 64 win team against a more difficult than league average schedule. Next year, they project like this:

The Thunder figure not to fall off too much, as they project here as a 60–61 win team*, assuming a league average schedule. Why won’t they fall off? Well, Serge Ibaka, Russell Westbrook, and Kevin Durant are just 24, 25, and 25 years old respectively. They all project to improve. Those improvements alone help a great deal with the loss of Martin. But the Thunder, in all likelihood, probably won’t be quite as good as they were last year. They may win just as many games, or more, than they did last year, depending on their strength of schedule, but again, they underachieved last year and played a tougher than average schedule. They will be, in all likelihood, from an objective perspective, a lesser club next year. But they will still absolutely be one of the very best teams in the league and one of the top contenders to win the title. Having two top 10 players in the league, each at just 25 years old, does wonders for your ability to weather the loss of role players.
*I projected Ryan Gomes as a replacement level player at -2.5 xRAPM. In 2011–12, he was a -4.4 in xRAPM for the Clippers, but he was hurt quite often and I believe it damaged his effectiveness quite a bit. The previous year, where he was much healthier, Gomes was a much more useful -1.7 xRAPM. I think Gomes will probably be closer to -2.5 than the -4.4 that he was in his last year in the league. However, if you project Gomes to be that terrible again, the Thunder still project as a 59 win team against a league average schedule.