Pompey’s hopes of progressing to the Papa John’s Trophy knockout stages were left hanging by a thread following a 2-0 defeat to Sutton at Fratton Park.
It was a Southern Group B contest that never really threatened to move out of second gear and was largely a dull affair.
But Isaac Olaofe managed to break the deadlock after the interval and the Blues were then reduced to 10 men.
Ellis Harrison picked up an injury soon after coming off the bench and with three substitutions having already been made, nobody could replace him.
The visitors from League Two – who secured the National League title last term – then wrapped things up through Louis John.
Danny Cowley made eight changes from the side that beat Sunderland in wetter conditions in the league 10 days earlier.
Mahlon Romeo, Sean Raggett, and skipper Lee Brown were the only players to keep their place, with the latter two joined by Paul Downing in a back three protecting Alex Bass.
That meant Reeco Hackett was utilised as a left-sided wing-back, while Haji Mnoga partnered Louis Thompson in midfield.
Michael Jacobs was handed an attacking free role behind the loanee front pairing of George Hirst and Gassan Ahadme.
To say there were not an abundance of chances in the opening 45 minutes would be generous to say the least.
There was a complete lack of any entertainment for the small crowd, with the multiple changes for both sides probably not helping matters.
Hackett sent a free-kick thudding into the wall following a foul on Thompson, who was providing plenty of energy in the middle of the pitch.
A Will Randall corner at the other end then caused problems, only for Joe Kizzi to nod straight into the arms of Bass.
Thompson soon found space to surge forward, but saw his drilled effort bravely blocked by Coby Rowe.
But that was about it in terms of noteworthy action, with neither team able to find any kind of rhythm.
And the Blues were dealt a blow just before the break, as Downing limped off injured to be replaced by Kieron Freeman.
Half Time: Pompey 0 Sutton 0
Cowley made another substitution immediately after the interval, with Mnoga making way for Shaun Williams.
It was Sutton who were first to threaten when the action restarted, although Bass was not overly troubled by Tobi Sho-Silva’s shot.
A cross from Hackett was then dropped by United keeper Dean Bouzanis, but the ball was hacked clear before Raggett could capitalise.
It was the visitors who broke the deadlock on 55 minutes, however, after Olaofe met a low cross from Ricky Korboa.
His first attempt was blocked by Freeman, but the loose ball ran kindly for the striker to fire into the net.
Harrison had been waiting patiently to come onto the pitch before the goal was scored and quickly replaced Gassan.
But his evening did not last long, with the striker getting injured in a challenge and limping from the pitch.
He tried to run it off before eventually disappearing down the tunnel and leaving the Blues a man light for the final quarter of the contest.
And Sutton soon made the most of their numerical advantage, as a Randall free-kick was nodded back by Kizzi for John to tuck away.
They might have added a late gloss to the scoreline, but substitute Richie Bennett twice finished wastefully inside the box.
Pompey (3-4-1-2): Bass; Downing (Freeman 43), Raggett, Brown (c); Romeo, Mnoga (Williams 46), Thompson, Hackett; Jacobs; Hirst, Gassan (Harrison 56)
Subs not used: Steward, Kaba, Hughes, Tunnicliffe
Sutton (4-4-2): Bouzanis; Kizzi, Rowe, John, Wyatt; Korboa, Beautyman (c) (Eastmond 74), Smith, Randall; Sho-Silva (Bennett 65), Olaofe (Boldewijn 61)
Goals: Olaofe 55, John 71
Subs not used: Palmer, Lovatt, Milsom, Ajiboye
Referee: Lee Swabey
Attendance: 2,490 (198 away fans)