I have a little insight into the mono situation. My Senior year in college I cam down with mono about halfway through our season, but it wasn't diagnosed for almost 3 months b/c the soar throat didn't happened until near "the end."
Granted it was a different sport, but I was a swimmer in college so I have some idea how it can affect athletes. My Junior year I won 2 events and got 2nd in another at our conference meet- including a conference record. My Senior Year I started feeling bad around Thanksgiving and couldn't shake it. I did okay in our meets, but not up to my standards and it was a roller coaster b/c I had very little energy. At conference I ended up 3rd, 2nd and 5th in my events, even where I was favored in 2. Right after conference I was correctly diagnosed and treated. A month later at NCAAs I swam much better and earned All-American honors, though I still underacheived a bit.
Basically, you can't go more than 1/2 speed for long periods of time, you have no energy at all, and are very lethargic in every area. In my case, I missed about 1/3 of out morning practices- 3x a week for 6 months- after only missing one the previous year. I just couldn't get out of bed, though I didn't miss any afternoon practices. Even in practice I just couldn't get going.
Watching Willie play this year, it makes a lot of sense and I wouldn't be surprised if he's had this for a long time. His lack of aggressiveness, except in short spurts which can happen w/mono, and lethargy just seems to fit. Last year there were not real effort issues with him and I think this had a lot to do with what has happened this year. Just my $.02.
