Really? He's an overrated coach who had only success with super team, otherwise he has flopped everywhere
He came to a struggling Roma team, didn't do wonders or improve the results but the club started playing a more attractive brand of football and was going in the right direction. Arguably, he started to build what would be a Roma team that five top three finishes in a row. He was a rookie and maybe it was too soon for him, but he left of his own volition and the fans & management were still behind him.
I have no idea about his Celta stint. All I know is, thanks to the internet, he took over a Celta team that escaped relegation by a single point and turned them into a top half club. This was a club that was just promoted two seasons ago and which just lost their star player Iago Aspas to Liverpool. He only left because Barcelona came calling. He accomplished all these while having the team play fluidly, and again left behind a strong foundation thanks to which Celta had two consecutive top half finishes, even qualifying for European competitions in the second year.
People think he only joined Barcelona because he's an ex-legend of theirs but in fact it was also due to his work with the B team. He took over from Pep Guardiola and in his second season the B Team was promoted to the second tier for the first time in over a decade despite losing a lot of players to the senior team. He then followed up that season with finishing third in the second tier, but of course he wasn't allowed to be promoted to the La Liga as Barcelona was playing there. So he ended up leaving for Roma despite still having a contract and being absolutely adored there. In fact, his last season at the B Team is that team's best finish ever in history.
In many ways, he was viewed as a spiritual successor to Guardiola, albeit a more practical one. Yes, he arrived to an already great Barcelona team and yes, it was super-stacked with the addition of Suarez, but the man surely delivered. He won the treble in his first year. He won the domestic double the following year. Also won the Spanish Super Cup, the UEFA Super Cup and the CWC, so just like Pep he won everything there was to win. These results came against Atletico Madrid at their peak (who were the defending champions when he arrived) and a Real Madrid team that would go on to three-peat the CL. Yeah sure Barça was stacked but so was the competition. In his third and final season he only finished three points behind the champions Real Madrid while still winning two more domestic cups. Sure he "bombed out" of two CL campaigns at the quarter-finals stage but it's not like Barça came any closer to winning the CL after he left -- he's still the last man that won them the CL. We all know how fickle tournament football is and how teams with the must-win-favorites tags come under immense pressure at CL games. PSG and City are recent prime examples of this. After just a three season stint, he left with 9 trophies, fourth most of any Barcelona manager ever. (Guardiola won 14 trophies in 4 seasons, Gamper won 12 in 15 and Cruyff won 11 in 8.)
He arrived at an aging Spain squad that bombed out of the group stage of the 2014 WC and the round of 16 of the 2016 Euros & the 2018 WC, after dominating NT football between 2008 and 2012. He called up many new players, built a great young core that's bound to improve and deliver better results in the future. During 2020 Euros they were arguably the most dominant team, and not just in terms of possession. In every single game, they bested their opponents in terms of xG & chances created and a bunch of other metrics. He did all this while reinforcing the footballing ideals that helped Spain dominate during that 2008-2012 period. Everyone agreed that they were super unlucky and really have been looking for someone like David Villa since he retired from the NT. There just aren't many strikers who can be part of a team with those ideals while still producing goals, and none of them are Spanish. Again during this tournament, they hit the ground running in that Costa Rica game, and then dominated against a Germany that had to avoid losing for the better part of the game. They kind of "threw" that Japan game for the purpose of bracket shenanigans, but again they dominated during the entire game. The Morocco game, however, was the "same-old-Spain" of 2014-2018 era: dominating possession while having no end product (not goals, mind you, but shots) to show for it. Having regressed to this pitfall, which is arguably the worst crux of the ideals Spain lives and dies by, he ended up leaving on a sour note. Still, as this youthful Spanish squad he built grows and goes on to achieve greater things, while holding on to those said principles, people will recognize his part in that success.
I expect him to return to club management after a hiatus. Given the correct opportunity, he'll show the world that he's still a world class manager.