Morecambe boss Jim Bentley said his side’s ill-discipline in the first half proved costly as they slipped to a second defeat of the season at Fratton Park.
The Shrimps suffered a torrid opening 45 minutes, shipping three goals and losing Andy Wright to a red card as Pompey won their first game in League 2.
Bentley said: “When you go a place like Portsmouth with a big crowd behind them you know you are going to have to weather a storm and we did that pretty well for 20 minutes or so.
“We rode our luck a little but the crowd were just going quiet when we conceded a first.
“Sometimes you have to give the opposition credit and it was a well-worked goal but then they scored a second and that was a terrible one to give away and really gave them the upper hand.
“To have just one defender against four attackers after one of your corners breaks down is indefensible and it was a goal no side should ever concede.
“From then we just lost our shape, we started giving the ball away cheaply in silly areas and when Andy Wright was sent off the game was largely over.
“We were much better in the second half but credit to the lads they kept their discipline and showed some pride but we have to learn and we have to learn fast because we all know this is going to be a difficult year and we need to be at our best as soon as we can.”