Not being much of a doctor myself, I did a bit of research on MCL sprains before eventually turning to injury expert Will Carroll of Baseball Prospectus and Sports Illustrated. Here's what he told me when I asked about sprained MCLs:
MCL sprains aren't as bad as ACLs from an injury standpoint. In fact, the current thinking by sports med doctors is not to repair unless they're already in there to fix the ACL. With a good brace, the stability isn't as big a problem. Getting him back by the season opener is a bit of a stretch, but if we're talking about a sprain rather than a rupture, it's not only possible, it's probable. (The less the strain, the more likely.) The biggest concern is for his lateral mobility. When he's moving to the side opposite the injury, there could be a problem. The other problem for a QB is that he's likely to get hit on the outside of the leg, putting more stress on the MCL. A brace normally protects this adequately.
The question, then, is whether Harris has a mild sprain or if there was any actual tearing of the ligament. If it's the former, he might just miss the season opener. If it's the latter, the injury prognosis becomes more complicated.
It's doubtful the public will be given much information on the severity of the strain, so we'll have to keep an eye on his rehab to gauge progress.