Claudio Ranieri admits he cannot understand why Tiago has not performed for Juventus, whilst confirming he is still hoping to sign Mohamed Sissoko.
Tiago looks set to leave Turin and Tottenham are keen to secure his signature if they can strike a deal with Juve.
And Ranieri admits he is mystified by Tiago's failure to make an impact following his £10million switch from Lyon last summer.
"Certainly we have made mistakes but in general our purchases from the summer have been positive," Ranieri explained to Radio Anch'io.
"Although it is undeniable that some of the players have paid a price for the burden on someone who puts on the Bianconeri shirt.
"And my biggest concern has been Tiago, a player I have followed since he played in Portugal.
"He has all the characteristics to be a great player and I cannot understand why he is unable to make the breakthrough here."
Tiago has started just four games in Serie A for Juve this term and his chances of a place in the first team will be made even more difficult with Sissoko's likely arrival.
Juventus had been leading the chase for the Liverpool star - but their interest appeared to cool when their president Giovanni Cobolli Gigli said his club would not make any signings in January.
However, Ranieri has confirmed that he wants Sissoko as soon as possible.
"Sissoko could arrive," said the former Chelsea boss
"He is a good player that could fit into the pattern of our team."