I think there was enough in that game to alert PL referees to what Ronaldo is all about, especially when United are seeking a winner or equaliser, he, like Vardy, is a very clever player. But a cheat.
It's not a question of whether they're alert, it's a question of whether they're arsed.
I think it might have been the 2-1 win at Anfield under Rafa, but I remember Ronaldo going down like he'd been shot, and the referee just laughed it off. No booking, even though he clearly knew it was a dive.
They knew what he was like, even back in the day. We like to hope the game has moved on in those dozen or so years, but we all know it really hasn't.