Technically, yeah it is always possible to come back to pre-injury levels. It takes a long time though. At his age, he's declining pretty rapidly anyway, and that doesn't just pause because he's injured and then restart once he gets back to training. It's going on while he's out. But not only is it to do with the physical injury and getting yourself fit again. It's getting accustomed to your knee being "fixed" but knowing it's not quite the same, trusting your knee and yourself with everything, and many other mental aspects. I've had the injury (twice now, going to have my second surgery in the summer sadly), and obviously I'm no professional footballer, but it took me about a year to get back to playing football, and then another year to actually getting to a level where I was happy with how I was playing and felt like I was at a good level. Another few months until I trusted myself crossing the ball with my left foot while running with it down the wing (left leg was my weaker leg). If anything, probably made it easier for me since I mainly used my right leg anyway so that leg wasn't affected (now I tore my right ACL so we'll see how that goes). I'm not sure which one Ibrahimovic tore, but at the professional level, you really have to be at peak fitness to deal with how physical the game can be and little nudges can throw you off balance and get you worried about re-injuring your knee. Especially in the premier league where there were pretty clear signs last season that Ibra was starting to feel the pace of the game and look more and more fatigued as the season went on.