It was a late challenge regardless, he didn't have a chance, and I agree with everyone, Messi should have been out of the game by then, but I guess Guardiola was not convinced yet...Barca was not really on the offensive at that stage in the game but you never know, Atletico was still hanging around... it would have taken a stupid mistake for Barca to lose that game but it wasn't a completely convincing win either.
The defenseman was asking for it at that minute of the game, and the reason why he had such a small chance of ever getting away with that is because messi was running into a full sprint and the ball was more towards his right foot, Ujfalusi came from Messi's left side and reached across Messi''s left leg and there was not doubt he had to take out Messi's left leg in order to ever get to the ball...so before Ujfalusi crushed his ankle he took out his left leg too.
It was sort of stupid on both parts, Ujfalusi and Guardiola.