Wyne Rooney ended his goal drought after scoring the opener in the 20th minute against the fifth best team in Belgium.
Rooney, whose recent form has not been impressive, scored the second and third goals, thus making it the first hat-trick of the season for United.
Herrera, who appeared in the pitch for the first time, scored a goal of his own in the 63rd minute which confirmed United place in Champions League.
United finished 4th in the premier league table last season, a position that required them a play off before they progress to Champions League group stages. They were pooled together with Club Bruges from Belgium.