It has to be a MP or HP tu I believe… Here is what you gotta do if you are having problem with the negative input. HOLD DOWN LP until you finish the HP TU input… THEN FADC. Basically you are holding down LP for way too long and letting it go at the wrong time. The other option, is you have to TAP the button really fast. But if you are just trying to get through the trial, holding it down a little longer will do you fine.
cr.lp into HP TU is easy, just hold cr.lp down (don’t release it) and you won’t get any negative edge, press HP and you will get a HP TU, guaranteed.
Thanks, but I can’t seem to make the TU come out at all when I don’t release the LP as soon as I normally do. I’m probably messing up the inputs because I’m concentrating on not letting go of the LP.
Not important anymore though, I finally pulled it off. Had to remap a few buttons to make it easier on my finger muscles, but I got it. 
Scary that some people say that Sagat’s challenges are among the easiest in SSF4. I’ve currently done T. Hawk, Hakan, Cammy, Akuma, Zangief and now Sagat, and the one trial I would not want to do again is Sagat’s 24. It felt more like luck than any of the other ones I’ve done.
Bro i have been trying this sagat Trial #24 for weeks and still cant connect after FADC to cr.MP to Cr.Lp to HP TU despite keep trying the above methods. any kind soul willing to advise me? i very keen to get his star as i a fan of him since SF2.
I have only completed all trials on ryu,ken,akuma,gouken,bison,Guy,Blanka and Dan so my fightstick skills is not a problem.
I have this one and fuertes #24 left. The technique I use that connects but i’m always to slow to FADC TU is that when you c.mp hold it until the punch goes back into his body release then tap c.lp it works about pretty often I just cannot finish the trial. Just my $0.02!
Ultra street fighter 4 sagat challenge 15 is impossible the ex tiger uppercut doesn’t connect