Signed from Darlington earlier in the summer, the Irishman is expected to take on the mantle of departed frontman Bayo Akinfenwa at Sixfields and will have had home fans purring with his display against the Championship side.
After setting up Michael Jacobs for the home side's second goal, Purcell made the score 3-1 before half-time before a strong second-half comeback from the Hornets.
Trialist defender Marcus Hall again lined up for the Cobblers as preparations for the new season stepped up again with just two weeks to go now before the big League Two kick-off.