The Blancos icon has been blown away by the transformation his old side have undergone over the past five months and is now backing them to sweep all before themReal Madrid can win the treble this season, though he feels that they cannot afford a single "bad day" in the Champions League.
Los Blancos are currently three points clear of city rivals Atletico at the top of La Liga, while they will face Barcelona in the final of the Copa del Rey next month.
Their place in the last eight of the Champions League has been all but sealed by a 6-1 first-leg win at Schalke and Hierro is confident that Carlo Ancelotti's men can claim a treble.
Indeed, when asked during an interview with Esports en Xarxa if Madrid could sweep all before them between now and the end of the season, the former Blancos centre-half replied: "Without a doubt. Real Madrid can claim the treble. I can easily see it [happening].
"However, in the Champions League, they can not have a single bad day. Madrid are now at an extraordinary level but do not rule out Barca, Bayern [Munich] or PSG, who could also be very dangerous.
"Madrid, though, are very strong, very powerful. They have undergone a complete change in five months and now have great potential."