Buzz Lightyear isn't a major attraction. It is loved by many, but it isn't in the same league as Test Track and Splash Mountain, which are such significant draws that they often are highlighted in Disney's advertisements.
I haven't read that Splash Mountain was to be in rehab as well. Where did you read that information? It appears that that isn't going to be the case. Regardless, even if it were to be the case, while it is unusual to have two major attractions closed for rehab at the same time, it isn't unusual for it to be the case in January. I believe I recall a time a few years ago when both Kali River Rapids and Space Mountain were closed at the same time.