He was great and one of my favorites later in his career, but as a casual United fan with less investment, i struggle to understand all the underrated talk over the years. Played for one of the biggest clubs in the world that were the strongest in his league for most of his career, only played in a less respected/ lower coefficient league (relatively) for the first few years of his career, CL wins and other finals, played for one of the big national teams...just a very highly visible and widely rated as excellent/great player in general.
The only way i can really see how he can be considered underrated is if you are in that huge club bubble of only discussing and comparing him to other similar size club contemporary legends like Zidane, Iniesta, Pirlo and Xavi who all had better international careers to go along with the great club one, so not surprising at all they end up with even more worldwide recognition. The players that might be genuinely underrated in a wider footballing context tend not to have been ones playing with the absolute elite clubs for year after year, especially when talent was more spread out. It's usually understandable why they are lower rated too, as they will often be less proven with not as many chances to showcase themselves at a level like CL and/or internationally because of being from a weaker nation.