Both players joined last month in time to join up with the rest of the squad on their pre-season tour of the United States, meaning it has taken until the first weekend of August for them to be officially unveiled at a press conference.
Castan joined the club from Brazilian side Corinthians in a deal worth €5 million, while Roma paid Genoa €2.5 million for a 50 per cent stake of the 21-year-old Tachtsidis, after he spent last season out on loan at Serie B side Verona.
Baldini was quick to praise both players when speaking at their Austrian pre-season training camp.
When asked about Castan he said: "Castan won a Brazilian league championship and then, just a few weeks ago, the Copa Libertadores – a competition in which he played exceptionally well and attracted the attention of clubs from all over the world.
"The negotiations to bring him here were a bit drawn out, partly because Corinthians wanted him to sign a contract extension with them, but things worked out well for us in the end.
"I'm pleased that after Aldair, we now have another Brazilian who can play in the difficult centre-back role for Roma."
On Tachtsidis, he said: "I think you probably know him yourselves having seen him last year in Serie B with Verona, where he made 37 appearances and scored two fine goals – though neither was patch on his magical strike in Chicago against Lubin [a goal he scored during Roma's recent tour of the United States].
"Panagiotis has a lot of strengths and these qualities are exactly what our coach was looking for. Zeman rates him very highly.
"It will be his first season in Serie A so he'll need a bit of time to adjust to top-flight football.
"Who does he remind me of? Instinctively I'd say Fernando Redondo, especially for the way he uses his left foot, but objectively speaking it's early days to be making such comparisons.
"As I said, the boy has a lot of quality and we hope he can fulfill the great potential he has shown."
Roma start their Serie A campaign at home to Catania on August 26.
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