Below level 4 2 of the reasons to buy NordGems include.

1. to train, sell & make cash
2. to reduce the number of players to replace next season (longevity)

1. Works well if you want to use the cash to buy non-NordGems ie buy 1 player, train sell then buy a few.
1. Would only work for buying new NordGems if you can sell them for more than the NordGem cost at next level. Takes a lot of doing and is not something I've ever tried (takes a knowledgable wheeler dealer type like Davy T)

2. This is the normal process where being a NordGem allows you to gain or near gain the 75sp required to maintain its level and get off the 'blinded by stars' 'replace every season conveyor belt. Some extra training will be required.

Ofc longevity is only useful if he's a decent performer.

In that case he's a poor performer I'd sell (I do that with any player [except my favourites where performance blindness can set in]), if not I'd take a step back to decide what I want to achieve when buying NordGems.