Well, the short of it is that injuries heal at different rates for different people. That's just the way it goes. When an injury happens, we are given the average time injuries take to heal, but that's just an average. If it takes on average 4 weeks to heal from an injury some people may heal in a couple weeks and other may take a month and a half. There may even be some crazy examples where someone feels better in just a couple days and other people take months to get right. We generally expect athletes to heal on the fast end due to their physical condition and the resources they have at hand. However, the most important thing is the healing ability of the body, something we can't really know.
Can we do any better knowing it's an MCL sprain? Well we can get a more specific average! Let's use this page as an example.
In the case of a first degree sprain, sporting activities should not be undertaken for about 3 weeks. For second degree sprains, the rehabilitation period will be between 6 and 8 weeks.Very binary isn't it? The injury recovery times don't even overlap. Either you are ready in 3 weeks or you are ready in 6-8 weeks. (We're ignoring the 3rd option because it involves a tear) What about week 4 or 5? Clearly some people heal during that period, right?
We know it's already been three weeks, but that doesn't automatically mean it's a 2nd degree sprain. It could just mean he's slow to heal from a 1st degree sprain. The Nats called it a mild sprain, which would seem to indicate the lesser degree (though I haven't heard what the doctors think. They just agreed it was a sprain) So that's probably it. Hopefully someone will get info on the severity of the sprain to confirm this.
This is all well and good but how long then will it take Rendon to heal? What's like the upper limit? Well, diving around into the internet and message boards and comments and the like... let's see... he'll definitely be ready to go within a year or so.
That too broad? Another week or another 48? Well this goes back to knowing the average time it takes to heal. That's a helpful number. But as a statistician I can tell you that for most basic statistical analyses you need two values, the average and the variance. The average tells you the middle point, the variance tells you the spread. Here are two examples of weeks of injury healing time.
Example 1 : 1, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 5
Example 2 : 0.5, 0.5, 1, 1, 2, 2, 2, 3, 3, 3, 5, 13
Both of those give you an average healing time of 3 weeks. In Example 1 though you are pretty well centered around the 3 week period (your variance is low). In Example 2 there is a decent chance you heal in a few days and a not crazy chance it takes you months to heal (the variance is high). I can't be sure because I lack data but anecdotally, knee injuries seem to resemble the latter example more than the former. A lot of people heal up as expected but a handful can take forever.
The more worrisome part than Nats fans beyond the "I don't know" aspect is the fact that I think being a professional athlete actually lengthens the time out here. You really want to make sure it is healed before you start doing athletic activities. For you or me that means light jogging. For Rendon, as far as we are concerned, this means going at close to 100% against other professional athletes. They won't be allowed to slowly get back into things, or at least we won't consider that time "being back". So Rendon and Joe Q Public could feel the same after week 6 but while Joe Q Public might say he's ready because he can start doing some stationary bike exercise again, Rendon can't say it because he won't be major league ready.
But cautious is good here. A reinjury would mean certainly more time off than the first time (that obviously wasn't enough) or worse, a more serious injury. Given Rendon has missed 3 weeks already, let's say he comes back in two more, then goes down quickly. You'd be talking mid-June, All-Star break for a return? The Nats have to be very cautious here.
I'm not a doctor. I haven't seen Rendon's workouts. We can only go on what we hear. But what I'm hearing is a guy that should take at least a couple more weeks off. Forget Opening Day, let's hope for April.