Sounds like a case of the stuck stats on the warrior follower (because you met them at such a low level especially) to me. I noticed Erik the Slayer getting knocked down too easily and checked...his health was supposed to be something like 450. Stats getting stuck had left it down at something like 220. Oops. Some console trickery later and he was back to being his usual damage sponge self.
You can try fixing the warrior follower without console commands, for the record. Get your hands on Wabbajack (if you don't have it already) and dismiss the follower from your service. (Be sure to take any stuff you gave them first--otherwise it goes walkabout.)
When they start to leave, whap them with Wabbajack. Do this in an isolated area and save beforehand...being isolated avoids other NPCs aggro'ing you, and saving beforehand is in case they turn into a dremora as opposed to a chicken.

Then give them a quick punch to change them back to normal. Grab them, rehire, and stats should be where they normally would be for whatever their level is.
FYI, followers
do have their own set level caps that not even Wabbajack (IIRC) will budge them past. Some are capped at 50, others as low as 30 or 25. J'zargo is the only follower who will scale up to level 81 with the player...unless you have mods.

It's also possible that your warrior follower capped out at 30 or so, and that's why he's struggling now at 40. Edit: Actually if it's Lydia you're talking about, she's capped at 50, so that's not it. Must be stuck stats then.