McGruder is slated to undergo surgery next week.
It wasn't clear when the 26-year-old McGruder suffered the injury. He played 25 minutes in Wednesday's preseason victory over the Washington Wizards.
Robinson sustained the injury while driving to the basket during a scrimmage in practice on Sept. 29.
The Brooklyn Nets exercised the 2018-19 contract options for D'Angelo Russell, Rondae Hollis-Jefferson and Caris LeVert.
Russell, the No. 2 overall pick of the Los Angeles Lakers in the 2015 NBA Draft, was acquired by the Nets in June. He averaged 15.6 points, 3.5 rebounds, 4.8 assists and 1.4 steals last season.
As reported earlier, Harris agreed to the four-year, $84 million extension on Saturday.
Allen departed the team this summer and signed a one-year deal with the New Orleans Pelicans for the veteran's minimum of $2.3 million.