Isn't the word "champion" in Italy given to a talented player who has proven his ability on field regardless if he has won anything?
more or less, yes. It's not necessarily a way to describe a player who has won a title but rather someone who can make the difference and take his team to another level ... usually, however, these players have already won something anyway.
I believe what Moggi means by saying that Aguero is not a champion is
not that he is not a good player but that he is not someone who can walk into Juve and transform the team as it is, like Ibra did with Milan (although in that case there were other factors too).
Based on what Moggi's had to say on this, I think he is implying that Juve need to notably strengthen the team in a number of other areas and not just rely mostly on one costly signing to make most of the difference by himself - Aguero is a good player but not 45 mil and 7-8mil/season good ... these are figures a team pays for a player, who should be able to carry the team on his back, so to speak, and Moggi doesn't seem to see Aguero as a player of such a caliber ... and frankly, neither do I, as much as I like him.
Personally, I perfectly understand why Juve fans are going crazy over a good, famed and exotic talent like Aguero but if Juve wind up having a not-so-good next season, it won't be because they chose Rossi over Aguero but because they failed to improve the team as a whole ... I think Aguero is, overall, a better player than Rossi but not that much better as to make choosing Rossi over Aguero the same as choosing Rossi over players like Ibra, Eto'o, Del Piero, Zidane, Kaka, etc, etc ... now that would be a choice that Juve would definitely end up regretting, if they were to make it.