The Brazil international will miss Wednesday's game due to injury, and he fears his team-mates will face a difficult task to stop one of the best players ever.
"Messi is a special player, certainly the best attacker I’ve played against to date, but I think comparing players from different generations is difficult. All I know is that he’s spectacular and already is definitely among the best ever," Thiago Silva said to fifa.com.
However, the centre-back is hopeful of Milan getting a positive first-leg result against the Blaugrana to take to Camp Nou next week.
"We have to be optimistic. We’re happy to be taking on a team that’s currently considered to be the best in the world. We know how strong they are and we respect them, but we’re also a club with an immense pedigree and a lot of European titles.
"Almost everybody sees Barcelona as favourites and I think that’s natural. So, it’s down to us to prove over these games that we’ve got what it takes and to show Milan are worthy of winning the European title again.
"Milan’s victorious Champions League pedigree means they always get respect, while the fact we’ve already played them in the group phase is undoubtedly a plus point. I don’t know if Barcelona are wary of us, what matters is that we do our bit and play our football."
Thiago Silva then went on to discuss Zlatan Ibrahimovic, and had nothing but positive words for the Sweden international.
"Ibrahimovic is a very intelligent player and there are very few strikers like him in world football. Not only does he cause problems for defenders with his impressive physical strength, he’s also excellent in a ‘pivot’ role, giving Milan a reference point up front and bringing people into play.
"Even for great players, things don't go well at certain clubs and that's normal. He’s now got the opportunity to play well against Barcelona in the knockout stages of the Champions League, and of course he will be extra motivated."