USP let a lead slip to suffer a 2-1 Wessex League Premier Division home defeat after conceding twice in the final 10 minutes.
Things were looking good for Grice's men after Cameron Quirke's stunning volley handed them a half-time advantage, only for Jack Nicholas' penalty and Mark Sampson's goal sent the hosts crashing to defeat.
In assessment, Grice felt it was right for him to take some of the blame for the loss. US Portsmouth' s players were handed the night off training last Tuesday as they were without a game and as a reward for a good showing in the defeat at AFC Stoneham.
However, it was a decision the USP boss was left questioning himself for after the Hamble defeat. Grice said: ‘I think I’ve got to take some responsibility for it as well, if I’m completely honest.
‘After a good performance at Stoneham, I gave them the night off training on Tuesday.
‘I think that bit us in the bum if I’m completely honest. I think we looked a little bit lethargic.
‘They’ve got busy schedules (the players) they’ve always got something going on Tuesday, Thursday etc.
‘I think I need to take some ownership for the performance as well - I think we should have trained on Tuesday.’
Quirke's 36th-minute stunner looked to have put the hosts on their way to a fifth Wessex Premier win of the season.
When goalkeeper Dylan Kramer kept out a Josh Rose penalty 10 minutes after the break following Obi Saidy's foul in the area, US Portsmouth must have thought it was their day.
But Nicholas slotted home Hamble's second penalty on 80 minutes after Conner Grant was adjudged to have handled in the area.
US Portsmouth were then dealt a hammer blow as Sampson converted a cross with two minutes left - and Grice was left angered by the defeat.
He said: ‘I say it again, these are games we should be looking at winning. I’ve been disappointed with performances and stuff like that (at times this season), but sometimes you’ve got to hold your hands up and say you were beaten by the better team.
‘They had more impetus than us in the second half but they weren’t the better team. There’s no point saying it at all.’