The Pilgrims suffered a 5-1 defeat at Oakwell, having trailed 5-0 at half-time.
United midfielder Josh Panton struck a second half consolation.
Boston: Sam Buckland, Aron Ealham, Oliver Houlston (Freddie Lound 65), Toby Cotton, Archie Bradford, Elod Nadasdi, Oliver Callaby, Josh Panton, Joe Todd (Lake Taylor-Peak 40), Sam Finch, Toby Foreman (Jack Dixon 55). Sub (not used): Alfie Harney (gk).