If you don't have reverse, but other forward gears, then its not the filter plugged. Believe me, you DON'T want to hear what your trans pump sounds like when its not getting oil.

Relax- You would have had to remove the cap first to slide it into place.
It would be obvious and not half way down tube.
I think mine actually came off in packaging and was loose already LOL.

When I bought mine it was loose in the box, also I could not easily remove the old plastic ring, it was stuck in the hole..
We are talking about the same black plastic plug thingy right?
I ask because I do not remember a tube, unless you mean the part protruding from the filter, also it would not be hard to put the filter on without that part, if you did not know it was supposed to be there anyway.
Either way I doubt much more could happen other than a tiny little bit of fluid not getting filtered, and that shouldn't be able to cause his problem.