Why say Mangekyo Sharingan ? Take away the normal sharingan eye from Itachi & let's see what he can do,he wouldn't be able to cast genjutsu.As for Taijutsu , without his normal sharingan,he wouldn't be able to fight people like Guy or 8 gates Lee.Ninjutsu= FireBall Jutsu.
Without his sharingan,he wouldn't have even defeated Kakash in part 1.
He would be able to fight them. Itachi is smart and there is plenty of other jutsus that he can use. He is also fast. Uchiha's are known to be able to use shushin speed. He is able to use fire jutsus and knows taijutse. He also knows how to do shadow clones and use replacement jutsu if he can't get away from danger.
It's not how much power you have in you, it's how you use your power. The question is can Itachi use his other powers to perfection without using the Sharingan? the answer is simple. Itachi can because he uses his head to fight rather than brute power.
One good example is Bee rushing straight ahead to Nagato with his Brute Power without first confirming his powers.
The only thing that could have possibly saved Bee is his sword. While nagato was absorbing Bee's transformed chakra, Bee's sword might have absorbed bee's transformed chakra too so that it can give back the chakra that his sword absorbed to bee and still recover.
The point is here, You won't see Itachi rushing an enemy foolishly like Bee did and that's a difference with a lot of people who fight even in real life. There are those who uses their head and there are those who uses just brute power and little head. The one who uses their head will win