that's a contusion from a fall combined with a minor abrasion from scraping against something like concrete. i've seen similar injuries dealing with people who've fallen onto curbs or something on the ground. a bat to the knee swung with force would impact a much broader area and we would see subdermal injury, swelling, and bruising discoloration throughout the injured area because of the crushed capillaries from such an injury. contusions rarely result in abrasive injuries unless the person was dragged on the injured area, scraping away tissue.
look at pics online (if you want to do that, i don't recommend it though) for knee contusions or patellar tendon injuries.
additionally the area in question, if hit with a bat, would be debilitating as that portion of the knee is where the patellar ligament is (anybody who's played contact sports can attest to this) and injuries there, or even swelling there is usually crippling for a few weeks at the least and usually appears as a swollen lump along the injury site.
Lucas is also picking at the injury and not letting it scab/scar to heal. this can easily lead to infection not to mention if he has untreated diabetes, he will have a harder time healing from bleeding injuries than typical people.
Lucas is enough of an idiot to think "ACAB" is the name of a protest group or something, from his earlier posts on the Floyd riots. something something ACAB handing out water, et c. he doesn't actually know what the acronym means and doesn't care to investigate.
i don't really see any indication of a plank to the head. the scalp is quite thin skin generally speaking and stretched around the skull, so significant trauma to it tends to easy tear it, creating some pretty horrific looking wounds, often needing staples/sutures. if the "injury" is on his forehead, it just looks like he rubbed it a little on some stucco to abrade it a bit. his face also doesn't look abnormal in the way of having bruising or anything.