Moody’s Investors Service late Monday said it placed its debt ratings on Boeing Co. BA, +0.09% on review for downgrade, thanks to a likely “more costly and protracted recovery for Boeing to restore confidence with its various market constituents, and an ensuing period of heightened operational and financial risk, even if certification of the (737 Max) comes relatively near-term.” News last week that major Boeing supplier Spirit AeroSystems Holdings Inc. SPR, -2.78% will lay off workers in connection with the Max grounding “was unexpected and is an example of the ongoing event risk weighing on (Boeing’s) credit profile,” Moody’s said. Boeing’s 737 Max jets have been grounded worldwide since March after two deadly crashes months apart were connected to a faulty anti-stall system. The airplane’s return-to-service date has been stretched for months. “Moody’s considers that the longer the grounding runs, the greater the risk to Boeing’s already blemished reputation,” the debt ratings agency said. Boeing’s senior unsecured debt is rated A3, one of the lower echelons of investment-grade bonds. Moody’s on Monday downgraded Spirit’s debt to junk territory on liquidity concerns. Shares of Boeing rose 0.1% in the extended session after ending the regular trading day up 0.1%.