IMO Taylor could be good for Allen, since Rivers wasn’t exactly captain ball security, and Taylor might zero on Allen as a security blanket
LOL at the notion that Taylor could be as good as or better than Rivers for Allen's production.
For purposes of this post, I will initially assume Tyrod plays the entire season.
The Chargers offense under Lynn has run 1020 plays (2017), 945 plays (2018), and 997 plays (2019). Before coming to the Chargers, Lynn was the assistant HC in 2015 and the OC in 2016 in Buffalo, with Tyrod as the starting QB in both of those seasons. The Bills ran 1016 and 1012 plays in those seasons, respectively. Ignoring the outlier 2018 season, that is an average of 1011 plays per season. So I am starting with that number.
I expect the passing/rushing split to shift more toward rushing than was the case with Rivers, but not quite as far toward rushing as those Bills teams. (I like Ekeler, but he is not Gordon or Lesean McCoy as a feature rusher.) So I will project 1011 plays, with 571 called passing plays and 440 called rushing plays.
In 2015-2016, Tyrod dropped back 997 times, was sacked 78 times (7.8%), and scrambled 103 times (10.3%). In 2017, under new HC Sean McDermott, Tyrod's sack rate was a bit higher and his scramble rate a bit lower. He is older and more mature at this point, and he will also be playing with a better group of receiving targets than he had in Buffalo, so it seems reasonable that he might scramble a bit less. On 571 dropbacks, I will predict 43 sacks (7.5%) and 48 scrambles (8.4%). That leaves 480 passing attempts.
If Allen gets 25% of the targets, that is 120 targets. But will he get that many? I doubt it. Rivers' strengths (reading defense, pre- and post-snap; throwing with anticipation; accuracy) are a really good fit with Allen, and none of those are strengths for Tyrod. I expect this means Tyrod will not attempt some of the passes that Rivers did, further reducing Allen's targets. I will assume 7 targets per game = 112 targets if he plays all 16 games.
His catch rate seems likely to come down for the same reasons mentioned above about Tyrod vs. Rivers. Throws to him will likely be less accurate. Assuming 65%, that results in 73 receptions.
His career YPR is 12.2. His career low season YPR was 10.2, his career high was 14.7, and last season, it was 11.5. Allen is typically in the top 15 WRs in YAC, but, again, that is facilitated by accurate throws that lead him away from defenders, and I expect that accuracy won't be there this season. I think 11 YPR is a generous assumption. On 73 receptions, that is 803 yards.
TDs are difficult to predict. Allen has 18 TDs on 444 targets over the past 3 seasons, which is 4% of his targets. That rate on 112 targets equates to 4.5 TDs. But Tyrod's career TD percentage is 4.0%, compared to Rivers' 5.2%. So I will go under and project 4.
So if he remains healthy and if Tyrod starts the entire season, I project Allen for 73/803/4 on 112 targets. Pretty underwhelming, and definitely not a top 20 WR.
All that said, I very seriously doubt Tyrod will start all season. I expect the Chargers to have a losing record entering the bye and expect Herbert to be starting no later than week 11 after the bye. Unfortunately for Allen, that seems more likely to reduce his production than boost it. But admittedly, Herbert is a wild card, so that is hard to say for sure.