There are many factors but ultimately power to weight/watts per kilograms is key. The solution is important, not the variables, ie: Naro vs. Tom at the GdI, Their weight and watts are different, but I bet that they are both in the 6 w/kg range. Note: I don't know what to 'trust' in determining this number for pros.
So Tejay: I think that when he gets down to the weight that he thinks a GC rider should be, his w/kg are at the tipping point. In training he can produce a good w/kg test so he thinks all is well. After several days of hard racing though, I think that his w/kg has dropped enough that he is no longer able to stay with the front group. Most of us here, me included, keep referring to how he is a better one week racer than GT guy. Now that I am reflecting on that, IMO, the reason he does well at those event is because he hasn't gotten down to his GT weight. All of his good results have come when he is heavier. Even his wins on US roads have come when he isn't GT ready.
Fair enough jmdirt.
I looked at Tom Dumoulin yesterday for a size reference.
He looks, for a pro cyclist, to have a decent amount of mass in his core. His core would be his shoulders, back and belly down to his pelvis.
His limb circumference looked relatively alright for a cyclist.
Just a quick observation, but in terms of having extra energy reserves, right now, 2 weeks into the Giro, he looks quite physically, without mentioning his actual riding, good to go.
What I am saying, is Tejay's base weight, that he rides the season opener at, needs to jump up at least 5kg.
His threshold and total power should go up. His watts/kg might stay the same, but his climbing ability will actually suffer on steeper climbs. But he is not great on them anyway.
Libertine discussed parcours on the last page...Tejay is not going to get a favourable outcome there.
He literally, has to change or adapt his bodies mechanical efficiency and increase energy surplus.

So he puts some muscle mass on.
I disagree that this can be done mid season. That's not easy to do with a full racing and training load.
RedheadDane...what you've said is the nucleus of why I hold BMC to blame. If they ran the probability of last year and the year before's Tour incidents occurring again, where would that have left Tejay if it occurred this season? In a really crappy position.
Last time I checked, he was 26 minutes behind his own domestique. Nobody wants to be in that position.
He should finish the Giro, then hopefully allowed to go back to the USA, maybe race Utah, then have a go at the Vuelta where the team reconsiders their plans after the Tour. In no way, should he make the Tour squad.
Based on BMC's sticking with him, in training or testing, Tejay is probably the strongest rider at BMC. Except, that's the form for starting a race in. Get to the end and that picture is vastly different.
Anyway...hopefully he gets better.