Injury and unavailability have forced Kenny Jackett to field a starting line-up with an average age of 22.5 years in their two most recent matches.
Within that scenario, there is a 21-year-old captain and a 26-year-old representing the oldest player in the squad.
During fluctuating fortunes last week, there was the impressive 2-1 win at Fleetwood, followed by an abject 2-0 loss at Blackpool.
And Pompey’s assistant boss admitted a broader age range was required to boost performances.
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Gallen said: ‘It’s unusual to have such a young team – but it didn’t seem that unusual at Fleetwood.
‘It also didn’t seem that unusual until the first goal against Blackpool and definitely the second goal. Then it didn’t look quite right at all.
‘It’s like anything else in football, you need to have a team.
‘You cannot field 11 30-year-olds, you cannot have 11 20-year-olds, you need the blending and mix to be right, with a proportion from each group.
‘You need some athleticism, people who can play, players with pace, you need some old heads, you need some youth.
‘Our team last Saturday was too skewed towards the younger end of things, especially with the first goal going against you, which then dampens confidence.
‘Had we conceded the goal but had Pitman, Evans, Thompson, then we wouldn’t have looked as youthful.
‘The best teams have a mix and blend of all ingredients required. You can’t be too young, you can’t be too old.
‘The blend and the mix have to be right for a successful team and on Saturday – on this occasion – it looked too inexperienced.’