There are far worse cards to spend 100 soul on than Emerald Rejuvenator. But as someone who plays Sajj (and especially loves Control Sajj)- it’s a finnicky numbers game. As a singleton, the Rejuvenator won’t come up often, but saving it in aggressive matchups will help you turn things around. Double Rejuvenator is solid, but the opportunity cost tends to reveal itself more- where you could be playing Saon, some tech or Dioltas to slow your opponent’s damage output down. At 3, you’ll tend to draw out the game in a way that some opponents will just reposition themself, and deploy a Jammer or some other means of building cardflow.
If played with other tools to mitigate damage (Falcius, Mystics, Healers, Grove Lions), your health will go much further, but the slots may be better spent with tools that win the topdeck war. So unfortunately, it all comes down to the context of the deck you’re playing, if and how many Rejuvenators would be appropriate.
If you provide a decklist, I’m sure that we could help you figure out how to stretch those precious points of health, so that you don’t have to get your face melted. But, bare in mind, that burst weakness is just a trait of Vetruvian as a faction- so it may just to be an angle that you have to tolerate and work around with careful positioning.
EDIT: One important thing to understand about Rejuvenator is that 4 ATK is a very nice thing to have, so if it compliments the rest of your threats, it could be absolutely solid. But if it fights with your other threats, it may lead to inefficient trades.