The NBA’s Board of Governors is expected to approve Bill Chisholm’s bid to buy the Celtics when it meets here this week. A group led by Chisholm agreed to purchase the team from the Grousbeck family at a valuation of $6.1 billion. Fifty-one percent will be sold once the deal is final, with the balance closing in 2028. Lead governor Wyc Grousbeck is expected to remain in control until then.