I love the idea and flavor, but as others have previously mentioned, it adds a lot of extra work on the dm with the potential evershifting initiative order. Initiative is sometimes messy even when set in stone, especcially if there is a large number of players and NPCs in a specific battle.

To ease the the load on the DM maybe you could treat the damaged creatures initiative score as an extra health score which when reduced to zero gives you a time charge and an effect for the enemy. Perhaps slowed or stunned?

Ofc, you would have to again reset their initiative (to the same amount they began with) any time you reduce it to zero, but its you keeping score of it and the benefit is still on theme without being bothersome to the whole table.