Don't know if it will help forum members, since they probably already know, but
if you simply want to use Mplayer for watching youtube without any hasstle such as (youtube-dl) (poor mans youtube) or other methods
that can be hasstle...
then instead, you can simply enclose your movie Link in qoutes. (when executing Mplayer) Explained below.
WHen i want to watch anything from Youtube ...... i go to any alternative youtube mobile web page and search my Topic....for example www.genyoutube.net
1) i search my topic of interest.... such as "Rasberry pi" topic on www.genyoutube.net
2) i Choose one of the available movies from the many pages.
3) i Click the choice from 2 above.
4) i Scroll down and right click the "3GP 144" option and copy it's link. (it's rather a long link)
5) i Open Mplayer.
6) i Put a few arguments into Mplayer such as "-zoom" "-ontop" etc etc and finally at the end "MY MOVIE LINK from 4 above" inside qoutes and
7) This gives you a reasonable quality FULL SCREEN youtube movie (by pressing F) while using 10 times less data than the same movie
offerred up from the regular youtube web page player.
you can mess around with many other arguments like "-crop" "-aspect" and many many others. To get a perfect draggable movie in
a small window ontop of other work you do. (or full screen)
If you like, you can go to the regular youtube web page and find some movie of interest....copy it's LINK
and paste that Link into the GENYOUTUBE seach bar. It will return that exact movie and you can scroll down to get the 3GP 144 link.
and then follow the same list of instructions as above from 4 onwards