Sorry to break the news, but you're letting the stat sheet obscure that which is easily observed by watching the game. Some stats are beyond dispute, such as goals and assists, but some other stats are the result of what someone believes and records as a "fact". Mainoo of last season offered much more as a midfielder on the ball than the Ugarte of this season, who offers vastly more than Mainoo in the ground duel. They're two different kinds of midfielders, which comes as no surprise to anyone. In a ground duel you want Ugarte there every single time. In the final third, I would much rather see the Mainoo of last season with the ball than the Ugarte of this season. His ability (former ability, as he's been off it this season) to emerge out of tight spaces, pick the right pass, take on a defender and convert the shot in a key situation -- such as in a FA Cup final for the match winner -- is substantially greater than Ugarte, who excels in ground duels -- a conclusion I reach relying solely by observation and not a stat sheet -- and picking out the right release pass.
We've become too obsessed with xG and other stats in evaluating a player's performance. We can all choose to take in the game any way we wish, but the old way is often the best way -- watching it. And in watching the game we can observe that Ugarte is a defensive midfielder, a midfielder who is proficient in ground duels and picks out good passes. Mainoo is a central midfielder who contributes (or contributed, if we must) more in the attack. But if we were to blind ourselves to observation and rely solely on these stats, one could easily be led to believe that Ugarte is the central/attacking midfielder and Mainoo is the defensive midfielder.
But let's be fair. United overall were crap last season and to suggest that Mainoo excelled in an otherwise in another otherwise poor performing squad might be deemed faint praise. However, it can actually be the case that a single player excels when everyone else around him performs poorly. And if you go over the match threads and third party commentary you will find consistent praise, at times very high praise, for Mainoo's performances. And high praise for his contributions -- "exceptional" -- to the national team, not just his club team, in the Euros. Paul Scholes, who knows more about being a midfielder than either of us, compared Mainoo to Zidane. (I have to see anyone compare Ugarte to a legend of the game.) Just to make sure I wasn't guilty of faulty memory I went back over media reports and my recollections were confirmed and in fact the praise for Mainoo by third parties was even more effusive than I recalled. But what is it about Mainoo that set (past tense for our immediate purposes) apart as a top class midfielder (surprisingly so, since he was still only a teenager), since he's not a prolific goal scorer nor prolific maker of assists? He's not a long ball merchant. He doesn't go on long dribbles. He just doesn't dazzle the stats man. What he does is emerge out of tight spaces in the middle and final third with the ball, brushing off opponents as though there were nothing more than a fleeting nuisance, with clever, albeit quiet, several touches of the ball on the dribble then advancing the ball into promising situations rather than settling for recycling the ball. This is a skillset we really don't see from Bruno, who though brilliant in his own way (which I'm sure the stats confirm) too often rushes passes and too often goes for the long ball when it's really not on. Ugarte breaking up play and finds a long pass (2.91 v 1.51) as that what you would often expect from a 6, but Mainoo is (or was, since we're comparing Mainoo of 23/24 to Ugarte of 24 so far) does not look for the long ball and is the better option as an 8 and Ugarte the better option as a 6 -- and if you asked either for their own assessment I seriously doubt either would disagree.