Excellent points by bjscumacher and probably the root cause. I checked your game summaries and if it's the LB I think it is, he wasn't starting at all anywhere on any series in that first game. Looks like you're running 4-3 and Nickel (to the point above, only 2 LBs on the field in Nickel). One other thing you might want to do is adjust the substitution level (Sub Level) higher for your 3rd LB (and 2nd in Nickel) so you can bring that back up in more regularly, if you aren't comfortable making him the starter. Also, plug him in as a definite starter for one half and then scale back his playing time in the second half. Should get him the playing time he needs.
One other thing I'd say is that I feel like players who got promised starts didn't complain as quickly in the past. Like it took at least 3-4 games for them to get upset. Maybe I'm wrong on that but now they get upset by the second game, which I think is way too early. But anyway, it is what it is.