The stock ended down 23.8% at $129.61, below the $138.72 it closed on the last trading day before Muilenburg's appointment as chief executive officer on July 1, 2015.
Boeing shares had gained more than 136% during his term that abruptly ended with his resignation on Dec. 23.
The coronavirus outbreak could hit Boeing's cash flow further, S&P warned, as airline customers cancel flights and defer aircraft orders due to a plunge in travel demand.
Possible delivery deferrals have also raised concerns of production cuts at Boeing's larger widebody aircraft, which had not been impacted by the 737 MAX grounding.
The ratings agency lowered its credit rating on the planemaker to 'BBB' from 'A-' and now expects 2020 free cash flow in a range of negative $11 billion to $12 billion, down from a prior estimate of positive $2 billion.
Jefferies analyst Sheila Kahyaoglu had last week lowered the price target on Boeing's stock to $300 from $350, citing the risk of a potential dividend cut, as the company looks to save cash.
The stock hit an all-time high of more than $446 on March 1 last year, days before the fatal crash of an Ethiopian Airlines plane that led to the worldwide grounding of the 737 MAX.
Aviation regulators do not have a specific timeline for the 737 MAX's recertification that will allow deliveries to begin and generate the much-needed cash for Boeing as its debt continues to mount.
Reuters reportedhttps://reut.rs/2w0F71T last week that Boeing is set to drawdown the full amount of a $13.8 billion loan, likely bringing its total debt to $41.1 billion, up from $27.3 billion in 2019.
"Markets are melting down, airlines are grounding fleets, and traffic has collapsed. This has all dwarfed the MAX disaster," said Richard Aboulafia, analyst at Teal Group.
Shares of rival Airbus SE's also fell as much as 24% in Paris to hit a more than three-year low.
Reuters reported on Monday Airbus had drawn up contingency plans to slow or stop production, if France is placed under a drastic lockdown due to the coronavirus.
Graphic - Boeing erases gains from former CEO Muilenburg's term:
By Ankit Ajmera