When you need to replace an engine at short notice it will often be a case of what is available, new engines are not often sitting on the storeroom shelf!
Nor do you leave old tired engines around for emergecy use and mix them with another tired engine already on the plane.
The idea to manage fleets is to avoid inflight shut downs, air-turn-backs and diversions as they simply waste resources compared to having ready 'good" spares available. Thus you cycle engines through shops ASAP in order to make them available as spares. It's a matter of spend now or spend a lot more later.
Spares are as-good-as-new and enough are available (even via lease) to not having to put an engine on that needs to be taken off before the next scheduled hangar check.