I met Semin a few times at the big fan events. Super nice, but also obviously extremely introverted. He avoided attention and the press like the plague, and he seemed to struggle with the language more as a result. But if you followed him between seasons and when he left for the KHL he was no more apt to talk to the press than he was here. They labeled him a quiet leader. Our press had nothing to work with so they just filled in the blanks and he became Alex The Enigma.
So when the "We're going to build a 2nd line around Semin" era began and lasted for years because GMGM couldn't find a 2C to save his life, Semin checked out. He had every right to be frustrated. He was a creative genius with the puck playing with linemates that couldn't keep up. His attitude left a lot to be desired for sure, but there's nothing enigmatic about a guy who's frustrated for reasons that are plain as day.
We tell a similar story in DC about Jagr; that those were just the years he decided to play worse. And just like Semin, while there's certainly some truth there with regard to his attitude, we also did jack shit to cater to or seriously leverage his abilities. Both guys struggled here for plenty of valid reasons. Attitude and drive were definitely part of it, but giving them more to work with would have helped.
So if Mantha is jaded for similar reasons, he should get over it because he doesn't have the skills to make dealing with this shit worth it. The similarities end with his ceiling.
Big big disagree with the Jagr and Semin comparisons.
Semin cared, he just had a reputation issue (in multiple respects). There was Western media shyness bit you already touched on, but he also tended to mostly congregate with the Russian players in the locker room, and also had a reputation for stick penalties with the referees. He would sometimes work miracles on the ice, but due to his reputation, the rest of the time he wasn't working miracles he was percieved to be loafing. His aggressive stick work began drawing penalties, and then those penalties were also classified as lazy play (although his defensive stick work was anything but lazy, and he was actually an excellent penalty killer - at one point he had as many SHG for as PPG against during a 2 season stretch). Without much rapport with reporters or even factions in the locker room, the reputations he got stuck and spread.
Jagr, on the other hand, absolutely did loaf it in Washington. He went from being a 121 point player in the year before joining the Capitals to player who would never score more than 79 points in a season for the Capitals (with his best pace being 93 points over a full season). He then returned to a 54G, 123P form after the lock-out in New York. Claiming Washington "did jack shit to cater or seriously leverage his abilities" is absolutely incorrect. After Jagr's first season in DC, the Capitals went out and got Robert Lang, who had been Jagr's center in Pittsburgh in the post-Lemiuex years. (And it's not like his center in his first year in DC was a sack of potatoes - it was Adam Oates.) Lang did well enough in DC that by the time of the firesale, he was not just leading the Capitals in scoring (29 points ahead of Jagr), but leading the entire NHL (Lang became the first player in NHL history to be traded while leading the league in points). Lang would fetch a fortune for the Capitals - in the form of Tomas Fleischmann, a 2004 1st round pick (Mike Green), and a 2006 4th round pick. The fact that Lang's return was so much more impressive than Jagr's (pending UFA Anson Carter - and that's it) speaks volumes regarding how both player's were perceived at the time. Jagr was a terrible contract, even for a league that was still without a salary cap at the time (and most knew was likely facing salary reductions in the coming labor deal). He was nothing more than a salary dump.
Beyond just Lang, the Capitals had plenty more for Jagr to work with. They had a future-HoF center for Jagr in his first season, in Adam Oates. They had two premiere powerplay threats in the league to compliment him, Peter Bondra and Sergei Gonchar. They had a checking line anchored by Steve Konowalchuk and Jeff Halpern to handle the difficult match-ups and heavy lifting deployments. They had won the division in each of the past two seasons. It wasn't a bunch of scrubs.
If going out and getting Jagr's preferred center, who he had past chemistry with, after he didn't click with one of the best playmakers in NHL history isn't "doing jack shit to seriously leverage Jagr's abilities" I don't know what is.
f*** Jagr, always and forever.