Ramos knows the Bulgaria star well from his year in charge at Tottenham, which ended 12 months ago on Sunday.
With Wayne Rooney absent injured, Berbatov could be leading the United line as they take on Ramos' current side in a Champions League clash in the Russian capital on Wednesday.
"The most dangerous player is Berbatov," Ramos said. "I worked with him at Tottenham, he is a great player.
"I can't say how he has adapted to Manchester United but when he is in good form he is absolutely unstoppable."
CSKA go into the Group B encounter at the Luzhniki Stadium with three points from their two games, three behind unbeaten United.
There is a feeling English champions United, missing a handful of key players and facing a Premier League trip to rivals Liverpool at the weekend, might be satisfied with a draw.
Ramos, however, is convinced Sir Alex Ferguson will have his sights set firmly on victory.
The Spaniard said: "Manchester United will play Liverpool on Sunday but I am sure the match with CSKA Moscow is very important for Sir Alex Ferguson's team.
"I think all the Manchester United players will do their best tomorrow.
"Some key players for United will not play but top clubs have not only 11 players, but much more.
"Sir Alex will find adequate reserve players, it is a great squad."
Ramos took charge at CSKA when he succeeded Brazilian legend Zico last month.
He believes the club has great potential and is pleased to see them sharing the same stage as teams such as three-times European champions United.
He said: "Sir Alex will make changes but CSKA will change too - both teams have big squads.
"It is a very important match because we want to show CSKA Moscow is a top club too. We will think only about the win."