Russell Westbrook and the Oklahoma City Thunder have reached an agreement in principle on a new contract potentially worth $85.7 million over the next three seasons, league sources told ESPN. The deal includes a player option that would allow Westbrook to still become a free agent after the 2017-18 season.

Westbrook, who has played all eight seasons of his career for the Thunder, will sign the contract Thursday in Oklahoma City, sources said. The Thunder announced a news conference on Thursday at 5:30 p.m. ET for what the team called a "major announcement."

Talks between the sides centered on the addition of two more years to Westbrook's current deal, which expires after the 2016-17 season. The new deal would call for Westbrook's salary to jump from $17.8 million next season to the maximum $26.5 million. He would then earn the max in the next two seasons if he exercises the third-year option.

The player option for the third season of the deal will allow Westbrook to capitalize on the league rule allowing players who have completed 10 seasons to earn a larger max salary.

According to league sources, Russell Westbrook will sign a new three-year deal with the Thunder on Thursday in Oklahoma City. Layne Murdoch/NBAE via Getty Images

In the wake of former MVP Kevin Durant signing with the Golden State Warriors, the Thunder had a new offseason objective: locking up Westbrook to a long-term deal. Oklahoma City removed its qualifying offer to restricted free agent Dion Waiters to free up cap space to offer Westbrook a max extension. Waiters agreed to a deal with the Miami Heat last month.

Westbrook had a season for the ages in 2015-16, tying Magic Johnson for the most triple-doubles in the past 50 years with a league-high 18, including six in the month of March.

A five-time All-Star and two-time All-Star Game MVP, Westbrook averaged 23.5 points, 7.8 rebounds and 10.4 assists last season and finished fourth in the MVP voting.