Sounds like just a strain, and that he might have played through it if the game was close. Despite being 1-3 they are far from being in big trouble in a division where 9 wins could take it.
Not sure there is any real evidence that he could have gone back in the game. Odd. I really didn't see anything in the play prior to when he came out.
If it is any kind of strain, I'd think they hold him out until after the bye.
That said, this team is in total disarray. The defense is horrendous and now even more riddled with injuries. If Lamar is healthy, they're going to need him. The game scripts would be more favorable for Lamar (and not so for D. Henry).
He didn't seem hurt too badly, it kinda seemed like more of a white flag thing than anything to me. Which I'm honestly fine with, I recall arguing that Washington should have done the same thing in week 2, but didn't and then Daniels (and Ekeler) got hurt.
He didn't go in the medical tent, or into the locker room, he wasn't being checked by trainers, he just came out. Until I hear anything to suggest otherwise, I'm expecting him to start this week.
Worth noting, Lamar was awful in this game, he and Monken had their worst game in the 2 years and 4 games they've been together. Just looked confused after the 1st drive, and watching it back this morning, it wasn't on the OL. Lamar had time, guys were open, it was like a self-fulfilling prophecy where he was expecting things to go wrong so then they did. Really indecisive, and Monken didn't help by basically just running the same stuff over and over. They also gave up on Henry and the run game WAY too quickly.
I'd just burn the tape and move on from this one. Ravens are in maybe the worst division in the NFL. 1-3 is not a death sentence at all. 9 wins can take this division. Browns have no offense, Bengals have no defense (and a very questionable offense) and Steelers are mediocre on both sides and are nowhere near as good as their 3-1 record that consists of barely beating the Jets, the Patriots, and a Vikings team that ran out of OL. Ravens defense is a major issue right now, but this is still a top-5 offense easily.
Assuming Lamar is ok-ish (like even 60% healthy) I think you have to start him. You need to beat Houston. That's must win. Home against a 1-3 team. Cooper Rush is awful (Mike McCarthy deserves a medal for getting anything out of him) there isn't a bigger starter to backup downgrade in the NFL.