The Pilgrims were beaten 2-1 at home by Guiseley, who ended the match with 10 men.
Luke Horne scored a late consolation for United.
Boston: Cameron Fox, Ellis Perry-Burdett, Josh Doran (Charlie Brooks 75), Will Langford, Finley Ging, Taylor Jaine, Jake Henderson (Nathan Bowen 65), Kris Zaluzinskis, Rueben Fogg (Flynn Slater 65), Hugo Hunt, Luke Horne. Subs (not used): Joe Wilkinson, Tyler Griffin (gk).
Referee: Trevor Basker.