let them play a couple of games and see how's their ratings.
whoever gets a 7 rating or higher , let them play on.

if it's lower then 7 , then after 2 games - just bench them and train them a bit then try again to incorporate them as subs after a few games and see how they do.

it sometimes takes a while - i had a 3 of them:
1 was good so he kept opening in the line-up ,
and the other 2 weren't good at their first couple of games - then 1 was better after being benched so he started as sub and made it to the opening line-up , and the other one was benched for the entire season but when he became 19 then he started playing better during the pre-seasonal friendlies so he got his chance in the leauge and did good.