The Royals have dual-signed the striker after failing to make his mark at Southern League Premier South Gosport Borough this season.
Lethbridge, 21, who was released by Pompey in the summer of 2020, has been restricted to just six Boro appearances this season following his arrival from Bognor last summer, with just one of those outings arriving in the league as a substitute.
But Portchester boss Carter is hoping he can bring the best out of the forward desperately lacking in match action this season.
Carter admitted his side have been over-reliant on leading marksman Lee Wort (27) for goals this season.
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But now the Royals are hoping to get Lethbridge fit and firing to offer a helping hand in that department as they look to charge up the Wessex League Premier Division table.
Carter said: ‘He’s got a great pedigree, he just hasn’t got himself going at Gosport for one reason or another.
‘We need to get him a little bit fitter. He’s been off with Covid this week so he’s had to self-isolate - he’s not ill - he’s had to self-isolate so he hasn’t trained.
‘We’ve got a good frontline, it’s where we’ve throughout the season, we’ve got plenty of defenders but come a bit short with goals over the season. We’ve relied on Lee Wort a lot this season.
‘It’s a view for next season as well, that’s what we’re looking to build for, he’s dual-signed (Bradley) so we’ll assess things in the summer.
‘Hopefully we can get him enjoying his football again, get him a bit fitter, and scoring goals.’
Lethbridge, who made three EFL Trophy appearances during his time at the Blues, is a doubt for Saturday's home league meeting with Alton as his Covid self-isolation period only comes to an end on Friday.
But other than that Carter has no fresh injury problems as Portchester will be bidding to extend their current run of seven matches unbeaten in all competitions.
n Portchester have been drawn at home to Southern League Premier South Hartley Wintney in the Hampshire Senior Cup quarter-finals. The tie is scheduled to take place at The OnSite Group Stadium on Tuesday.