Usually when a witness is not believed by a jury it is a situation where the witness is not an eye witness to a crime. Here, McQueary testified that he saw Sandusky raping a 10 year old boy. In 99.9% of cases, a jury will accept an eye witness account as truth unless there is some evidence presented that discredits the eye witness, e.g. the witness has something to gain by incriminating the defendant (this is not the case with McQueary). The only way I see the defense team discrediting McQueary is by proving that he lied. Juries just don't randomly discredit a witness without a reason. Here, there is no reason to discredit McQueary's testimony.
Also, keep in mind that there are two former PSU employees that the prosecution is planning on filing perjury charges against because the prosecution did not believe their grand jury testimony. In that situation, the prosecution did not believe the two former PSU employees because their testimony contradicts McQueary's testimony. The prosecution has not proved that they were lying, but the contradictory testimony has lead the prosecution to believe that they are lying and are pursuing perjury charges. That shows you how much the prosecution believes in McQueary's testimony. Accordingly, with respect to McQueary (and you have to read the grand jury testimony to realize that this is pretty much a slam dunk case against Sandusky) if the defense is able to discredit McQueary it will only be by showing that McQueary lied. That is my point, based on McQueary's grand jury testimony, there really is no way to discredit McQueary other than by showing that he lied.