The in-form Frenchman dazzled once again for the Irons.
The in-form French playmaker opened the scoring in superb fashion on 68 minutes by beating David de Gea from at least 30 yards at Old Trafford as the Hammers looked on course for the semi-finals, before Anthony Martial equalised late on.
Prior to his free kick, an already-booked Payet appeared to go to ground rather easily in the Red Devils' penalty area, with referee Martin Atkinson failing to produce a second yellow card.
As a result, former West Ham striker Hartson appreciates the quality of the 28-year-old's goal, but claims that he probably should not have even been on the pitch at the time.
"Great game. What a wonderful free-kick from Payet, but he was lucky to be on the pitch for a dive a few minutes earlier and that would've been his second yellow," he told BBC Sport.
"For all of Man Utd's pressure it was Martial who got the equaliser and they probably deserved it. It is back to Upton Park we go for the replay."
The strike was merely the latest in a series of brilliant Payet moments this season, with that being his 11th goal of the season in all competitions.
Along with Leicester City duo Riyad Mahrez and Jamie Vardy, it would be a huge surprise if the former Marseille man is not in the running for PFA Player of the Year.