So I like to refrain from giving specific medical advice without actually examining people, but I can tell you some generalities for your, um, education.
ACL and MCL injuries usually don't happen without a specific injury. Patients typically come in saying they got hit or torqued funny and felt a pop and some pain. Generally, meniscus injuries occur the same way. Stiffness would be a rare presentation of either injury, as meniscus is usually painful and cruciate ligaments are usually decreased stability or feeling like your leg is giving out underneath you.
"Stiffness" can mean a lot of different things depending on the person. If a guy comes in with actual decreased mobility and I feel greater resistance to flexion or extension, it's almost always a result of adhesions within the capsule which would be treated with arthoscopy, a very simple procedure.
If a patient comes in saying they just have increased pain and soreness after exercise(which sort of sounds like what you're saying), its likely to be an inflammatory process. I would start them on NSAIDs and if it progresses or doesn't improve, would consider a steroid shot in the knee.
When you say it's on top of the patella, what do you mean? Top as in closest to the skin or top as in closest to your head? The former would be suggestive of possible bony involvement, and the latter would make me think something more like quad tendinitis or something of that sort.
Regardless, the answer is likely going to be ibuprofen + rest + ice and possibly steroids if it persists or progresses.