What were you predicting the 2019 passing game to look like in April of 2018, coming off back-to-back 580+ attempt years?
What were your target projections for Henry before it was known he was going to miss the season?
I searched to see if I could find any Chargers/Rivers/Henry projections I made last offseason and couldn't find any. Not sure if I posted anything or not, since the search function here isn't the most helpful.
If I had projected the Chargers team targets for 2018, I think I would have considered the team targets for 2017, in Lynn's first season as head coach: 583 passing attempts - 35 throwaways - 11 batted passes - 3 passes on which Rivers was hit as he threw - 2 spikes = 532 team targets. I would have probably reduced that by 1-2 pass attempts per game due to the expectation that the defense and running game would be better... perhaps by 1.5 attempts per game. Assuming the same number of non-target pass attempts (~8.7%), I probably would have projected 510 team targets. (All numbers from PFF.)
That would have missed high, since the actual results were 512 passing attempts - 17 throwaways - 13 batted passes - 7 passes on which the QB was hit as he threw - 0 spikes = 475 team targets. It was hard to project that the Chargers would run 75 fewer plays in 2018 than 2017, despite 20 fewer rushing attempts... or that the rushing yards per carry would go from 3.8 to 4.7. In retrospect, I think Lynn was much more comfortable in his second season as head coach, and he imposed the offensive philosophy he prefers. This is why I think the 2019 offense will be more like 2018 than previous seasons.
Given that I may have projected 510 team targets, what might I have projected for Henry before his injury? Well, for one thing, Gates wasn't signed at that time. So I would have projected Henry as TE1, Green as TE2, and Culkin (who ended up not making the roster) as TE3. My guess is that I would have projected Henry for 80 targets, Green for 25, and Culkin for 10, or something like that.
But keep in mind, entering last season, Mike Williams had not broken out. Now he has. Ekeler also had not yet solidified his role as a strong target in the passing game. Now he has. Those things affects TE targets. So, even if Henry was healthy, I think I would have missed high on that projection.
It is also worth noting that Henry is a very good blocker, and that can work to his detriment. Chargers fans, myself included, were frustrated that Henry didn't get more targets in 2017. The general reason for it was that the team felt it needed to keep Henry in to block often enough that it held his routes and targets down.
That is my best answer to your question. Perhaps your question was intended to imply that someone who follows the team closely cannot necessarily project outcomes accurately. I'm sure that is true, at least for me. Projecting NFL outcomes is pretty difficult, at least for me. But I stand by my point that one who follows the team closely can project outcomes for the next season independent of injuries better than one who simply uses formulaic projections based on 1, 2, 3, etc. year sample sizes.