The Pilgrims were beaten 5-2 by EFL side Mansfield Town, despite forcing the match to extra-time at 2-2 and having a one-man advantage following Freddie Pitts' 90th minute red card.
Elliot Foster had given United a first half lead, with Pitts equalising before half-time - before Deakin and Ben Robson traded second half goals. Two further Deakin efforts in extra-time - to complete his hat-trick - sealed the deal for the Stags, before a last-gasp fifth from Max Hurdis.
Boston (3-4-2-1): Charlie Wilkinson; Bailey Dilley (Taylor Jaine 106), Owen Sheriff, Ben Robson; Krisztian Gagyi, Mackenzie Burdass, Joe Melson (Charlie Earth 94), Zane Millar; Han Stevens, Elliot Foster (Jake Henderson 104); Sam Harris (Logan Tate 68). Subs (not used): Mark Garner, Finley Rodgers, Cameron Fox (gk).
Referee: Andrew Hitchcox.
Attendance: 537.
PIC: Russell Dossett.