Robert McNamara has made some controversial decisions during his career in the Kennedy/Johnson Administration. He canceled the DynaSoar and made other goofs.
For example, In July 1967 General Westmoreland requested an extra 200,000 troops over and above the 475,000 due to arrive. In OTL Robert McNamara managed to persuade LBJ not to send these extra troops and also managed to persuade him to ignore demands from the JCS that the war should be extended to Laos, Cambodia and North Vietnam.
Let's just say that Robert Lovett (JFK's first choice, but he refused) decides to take the job of Secretary of Defence.
What would be the consequences?