Cooke, also the fund's portfolio manager, had been integral to the management of the fund since he took over portfolio management responsibilities in August 2007, the research house said.
"S&P considers Cooke's departure to be material and while GSAM has been quick to provide an updated structure of the business, Cooke's loss cannot be downplayed," S&P fund analyst David Erdonmez said.
It is understood GSAM will not replace Cooke, but instead will divide his responsibilities among its current staff.
The firm will also combine its fixed income and multi-sector investment teams, which will be led by GSAM head of cash and fixed interest Subash Pillai.
Pillai will take over responsibilities for the income fund.
Pillai had been part of the asset allocation committee for the past six years but had not been intimately involved with the day-to-day management of the fund, S&P said.
"We have been and continue to be concerned by the departure of senior personnel from the business, particularly given that they are not being replaced by seasoned investors," Erdonmez said.
Zenith Investment Partners said it did not believe the changes would have a material impact on its rating of the Core Plus Fixed Interest Fund, because the fund had a greater involvement from Goldman Sachs' offshore capabilities.
It continues to rate the fund highly recommended.
