In a game of few clear-cut chances Fortune's turn and shot eight minutes from time was enough for the Blues to see of the O's who suffered their first defeat in the new-look competition.
Cavasin made six changes to the side with Sam Sargeant, Aron Pollock, Robbie Weir, Ulrich Nnomo, Jordan Bowery and Ollie Palmer all starting, while defender Alan Dunne was named on the bench for the first time this season.
After a slow start the O’s created the first opportunity of the encounter, courtesy of a quick breakaway from Ulrich Nnomo. The Frenchman played a lovely throughball to Gavin Massey and after a couple of touches his effort was deflected behind for a corner.
Orient remained the likeliest to score in the opening stages as a free-kick was nodded back across goal by Aron Pollock and his centre back partner Tom Parkes fluffed his lines and shot wide.
Chances were few and far between in the opening exchanges but former O’s loanee Luke O’Neill was presented when he was unmarked on the edge of the box, but he flashed his effort wide of Sam Sargeant’s goal.
Sargeant was called upon 34 minutes in as Ben Coker’s deep free-kick found the head of Ryan Inniss who rose tallest in the box and at full stretch the O’s youngster pulled off a solid save.
The oressure was starting to come from the hosts as the O’s headed Ryan Leonard’s long throw behind for a corner, and then Parkes got a decisive touch to prevent Marc Antoine-Fortune from having a certain shot.
Sargeant was again in the action as he could only parry Simon Cox’s shot into the path of Leonard but from close range the midfielder fired wide, before Worsdsworth saw a shot from the edge of the area deflected behind for a corner.
Cavasin opted to make two changes at the break, with both Alan Dunne and Jens Janse coming on for Tom Parkes and Gavin Massey respectively.
It was Orient who created the first opening of the second period when substitute Janse’s fine ball in was met by the head of Nnomo, however it was just over the bar.
The O’s certainly looked lively and a crossfield ball found Janse once more, who laid the ball into the path of Jordan Bowery but his shot was tipped expertly wide of the post by Blues goalkeeper Mark Oxley.
The introduction of Dunne and Janse was proving a profitable one, as both combined to create another opportunity for Nnomo however his header struck the side netting.
Just as the game appeared to be heading towards a penalty shoot-out, the hosts took the lead when Fortune was allowed the time to turn in the box and fire into the far corner despite Sargeant getting a hand to the effort.
Jay Simpson was brought on in place of the injured Robbie Weir late on as the O’s pushed for a late leveller, however, it wasn’t to be and O’s boss Cavasin started his reign with a defeat ahead of his first league encounter against Portsmouth on Saturday.
Southend United: Oxley, White, Coker, Wordsworth (McLaughlin 48), Thompson, O’Neill (Bridge 81), Cox, Antoine-Fortune, King (Atkinson 59), Leonard, Inniss.
Subs: Smith, Mooney, Sokolilk, Kyprianou.
Goals: Fortune (82)
Booked: Fortune (41)
Leyton Orient: Sargeant, Hunt, Parkes (Dunne HT), Pollock, Kennedy, Massey (Janse HT), Weir (Simpson 87), Kelly, Nnomo, Bowery, Palmer.
Subs: Cisak, Koroma, Alzate, Benedicic.
Booked: Kennedy (54)
Attendance: 2,383 (413 Leyton Orient fans)
Referee: Andy Woolmer.