After seemingly being too good to tank at the start of the 2020-21 NBA season, the Oklahoma City Thunder won just two of their last 25 games to finish with a 22-50 record. That gave the Thunder the fourth-best odds in the 2021 draft lottery, but that translated into just the sixth overall pick.The Oklahoma City Thunder must put the disappointment of the draft lottery behind them and focus on the overall picture. GM Sam Presti has already resumed work on the rebuilding project by trading for Kemba Walker, in a move that netted OKC the 16th pick in the 2021 NBA Draft from the Boston Celtics.More offseason moves on the cards for the Oklahoma City Thunder?While the Kemba Walker trade was a good start, the Oklahoma City Thunder still have several holes to fill in their roster. They also need to acquire or consolidate assets that will improve the team in the long run. On that note, here are three needs that the Thunder need to address in the upcoming 2021 offseason.#1 Add big men to the rosterAl Horford finishes a dunkThe Oklahoma City Thunder sent centers Al Horford and Moses Brown to the Celtics in the process of acquiring Kemba Walker. Mike Muscala is becoming a free agent this summer, while Tony Bradley is entering restricted free agency.The Celtics are trading Kemba Walker, the No. 16 overall pick in the 2021 draft and a 2025 second-round draft pick to Oklahoma City for Al Horford, Moses Brown and a 2023 second-round pick, sources tell ESPN.— Adrian Wojnarowski (@wojespn) June 18, 2021This could potentially leave the Thunder without any recognized big men on the roster for the 2021-22 season. Isaiah Roby played as a stretch five in several games, but he simply doesn't have the size or physicality to become a regular at that spot.The Oklahoma City Thunder will have to attempt signing centers this offseason. They could re-sign Bradley, but he doesn't seem like a part of the Thunder's future at this point. Muscala has expressed his desire to return to the Thunder, though.❝It made it just really gratifying to come in every day and go to work.❞End-of-Season Interview with Mike Muscala 🎥 pic.twitter.com/BHdTPTvXK2— OKC THUNDER (@okcthunder) May 17, 2021OKC could also look to select a big man in the 2021 NBA Draft. They could've looked at drafting USC standout Evan Mobley, had their pick fallen into the top three. But since that isn't the case, the Thunder will have to look elsewhere. The likes of Alperen Sengun and Kai Jones could be potential targets.