Pompey rarely tested back-up Brighton goalkeeper Jason Steele at the Amex Stadium and could easily have been on the end of a far more embarrassing scoreline.

Graham Potter's hosts will travel to Championship club Preston in the next round.

Following Monday's defeat to Chelsea, Albion boss Potter changed his entire starting XI for the second-round tie and filled his bench with inexperienced youngsters.

Holland defender Joel Veltman, a summer signing from Ajax, was handed a debut, while keeper Steele was brought in for his first senior appearance since January 2019.

The raft of changes never looked likely to backfire as the Premier League hosts dominated possession all evening.

Argentina midfielder Mac Allister made the breakthrough in the 38th minute.

The creative 21-year-old, who has spent time on loan with former club Argentinos Juniors and Boca Juniors since signing for the Seagulls in January 2019, arrived unmarked to nod in Jahanbakhsh's pinpoint delivery after Marcus Harness had sloppily conceded possession.

Albion had been knocking on the door for a while at that point. Gyokeres forced Pompey keeper Craig MacGillivray into a smart early save, Pascal Gross had an effort headed off the line by Jack Whatmough, while stand-in Seagulls skipper Dale Stephens struck the base of the left post from distance.

Portsmouth, who came from 3-1 down to beat Stevenage on penalties in round one, showed three changes from Saturday's goalless draw with Shrewsbury.

Kenny Jackett's men had come close to an early opener when Seagulls defender Dan Burn almost diverted Ronan Curtis' 16th-minute free-kick into his own net.

Just before the break, Albion new boy Veltman inadvertently blocked a goalbound shot while lying injured inside the hosts' six-yard box as Harness attempted to atone for his costly error.

Any hopes of a fightback were quickly extinguished early in the second period.

Jahanbakhsh, who has been a largely peripheral figure since a £17million move from AZ Alkmaar in 2018, gave Potter a reminder of his talents by lashing into the roof of the net from just inside the box at the end of an incisive Albion breakaway in the 54th minute.

Brazilian defender Bernardo emphatically nodded in his maiden Seagulls goal from a Gross corner just three minutes later to take the game away from Jackett's men.

Sweden forward Gyokeres completed the scoring 19 minutes from time, easily turning James Bolton to slide under MacGillivray.

Steele was finally forced into a save inside the final 10 minutes, repelling a powerful header from Sean Raggett to ensure it was a fruitless trip along the south coast for Pompey.