No. It isn't. I use automatic and have won many races in legend.
What car do you drive?
Do you use assists? Which ones?
How faster are you than the professional difficulty?
Well, Automatic gears don't make the difference this year. 9 tenths slower isn't that bad. Just don't psyhically break thanks to it. Know that your team mate is faster and push harder but NEVER try to magically gain 1 second. I want a team mate that's faster than me. You get a lot better when you have someone faster than you. Just keep playing, if you do long races 50%+ - by the end of the season you probably will be faster than him.