Effient is a very potent anticoagulantia (often called bloodthinner). When a stent is placed, you need anticoagulantia to avoid thrombosis in the stent. Dependant on the type of stent you need to be on this medication for a couple of months. For a DES stent they normally keep you on it for a whole year, as for bare metal stents(BMS) the period is much shorter. It is very risky to stop Effient before the planned end date. An in-stent-thrombosis is often deadly. Your cardiologist will tell you when it is safe to stop Effient.
Did you discuss stopping Effient with your cardiologist? If not, I would strongly advice you to contact him ASAP. Note that there are other medicine with almost the same effect that might give you less side effects like Clopidogrel and Ticagrelor.
Your knee will heal just fine on effient but obviously a bit slower. Once clotting has started, then the process will speed up. If you had surgery on your knee 2 or 3 days ago, or longer, then it's fine to restart effient.