Boro went 2-0 in the first period with goals from George Saville and Britt Assombalonga before Newcastle turned things around.
United made eight changes during the second half, with Yoshinori Muto scoring a goal against Jonathan Woodgate.
Miguel Almiron beat the offside trap to level the game before Joelinton made the winning effort to seal a five-goal game.
Bruce will sense that the runout has added vital minutes to the legs of a United team that last played in March in Southampton.
The game was played at a good pace, but was not considered a training match until Toon’s coach Steve Harper blew the referee’s whistle and the staff members started.
How they aligned
United: Dubravka, Krafth, Schar, Fernandez, Rose, Ritchie, Hayden, Shelvey, Lazaro, Astu, Muto.
Appearances in the second half of; Darlow, Almiron, Yedlin, Lejeune, Lascelles, Manquillo, ASM, Bentaleb and Joelinton.