Ahead of tonight's Champions League qualifier, Donati said: "I am not too heavy. Look at me. Am I carrying any fat?
The former AC Milan midfielder also told the Evening Times: "Reaching the group stage is the first objective for the year.
"For me, and for the club, it is very very important.
"I am looking forward to playing in the Champions League. That is why I came to Celtic.
"I am 26 now and I want to play in an important competition like this."
Donati is delighted that standing shoulder to shoulder with him against the might of Spartak will be Scott Brown.
He said: "Scott is a fantastic player.
"I didn't know him before I came here. But he is a very energetic player. He is also intelligent and has a fantastic future.
"I like playing with him in midfield because I can sit back and play balls while he can use his energy to get forward."