Fabbrini joined the Championship side on a permanent deal during the January window.
The Italian joined Gary Rowett's side for £1.5m during the January transfer window (via Mail) having enjoyed a loan spell with the Blues towards the end of last season. And Caddis explained that once Fabbrini is up to speed, he will be a key player for the club's promotion charge.
"I think the manager has chased him for a while and has obviously got him," Caddis told the Mail.
"Diego is one of those players, he will win points on his own at times.
"It will be good for him when we get a run of games. I think he will be delighted if he plays next week."
The 25-year-old made his debut for the side in their goalless draw with Bristol City last week, playing around half an hour at Ashton Gate. And while he was unable to help the club to their third successive win, he showed during the loan spell he enjoyed with automatic-promotion chasers Middlesbrough that he can be a valuable member of a squad attempting to get into the Premier League.
He scored four goals and contributed two assists during this time at the Riverside Stadium. That level of creativity could be a very useful addition to Birmingham's squad. The Blues' current top goalscorer has just six strikes this season, and if they hope to remain in the promotion mix, they will surely need Caddis' assessment of Fabbrini to be accurate.