I assume Tellez would be able to just elect free agency if he were DFA'd.
The main issue I see with keeping him on the roster right now is the obvious one: there's no point in doing so if you have already decided to move on. That means he has to get some more playing time at some point, and at least to some extent consistent playing time, even if the leash is now very short.
He's not in the lineup tonight, so presumably we're in the "clear his head" stage. I think we're going to miss Kyle Harrison in this series, so I imagine he is back in tomorrow or Sunday. Maybe we'll draw Sale for one of the Braves games, and then there's potentially Kukuchi and Skubal at least on the road trip. Him being totally unplayable vs. LHP and all those guys being good compounds the problem (him being totally unplayable in general) even more, but I would figure he has this home stand and the road trip to show some signs of life now.
It would feel sudden to make the move otherwise, and I can't see any trade (for a replacement) happening soon. I guess their approach is that if he sucks, we can try Lamb out in a few weeks and then perhaps look to make a minor buy if the team is still winning.
Anyways, I think it needs to be said that this is the kind of thing that blows back on you when you cut corners and don't invest in the team intentionally. He could have been a cheap source of decent power, but instead we are paying 3 million dollars for nothing at one of the primary offensive positions, which we have no internal options to replace and which has been an issue for years and years and years.