The 23-year-old defender has impressed for the Magpies on their return to the Premier League, scoring winning goals in each of the club's last two games.
Lascelles has helped Newcastle to fourth in the fledgling table following their rocky start to the campaign and has been touted as a potential candidate for Southgate's senior squad.
Although Benitez insists that the former Nottingham Forest man definitely has the potential to break into the Three Lions squad, the Spaniard is keen to keep his player's feet on the ground.
The former Liverpool boss told Sky Sports News: "I [took him to one side for a chat], but I've done this with some players. When you are doing really well, I say to them it's important to concentrate on their jobs. If they are doing well, what they have to do is keep doing the same things.
"I have years of experience. It's the most difficult thing. If you are doing well and everyone is praising you and praising you, sometimes if you are young you lose your focus. It's to remind them that this is the way to continue being praised.
"He's happy with this approach. It's quite easy for me to tell him, 'You have to keep working hard', because he was doing it anyway. We're trying to improve other things in his game, but in terms of this - his mentality and character - he's fine.
"He's someone that has the potential and has to keep working the same way that he's working, and he will improve. How much will he improve? I don't know. But he has the potential and that's quite positive."
Lascelles has represented England at all youth international levels from Under-18 to Under-21, winning 12 caps in total.