If tennisabstract's 1990s data is accurate, then Goran might not be the right person to help Cilic. His career numbers: 54.9% of 1st serves made, 82.5% win% on first serves, 50.1% win% on second serves.

First of all, those numbers are crazy. That 1st serve has Cilic-level (bad) accuracy, Raonic-level (amazing) dominance when it goes in, and his second serve was somewhere between mediocre and bad. Goran was arguably not a top-100 player at any time during a match except when his first serve went in, when suddenly he jumped to the #1 player in the world.

The early returns for Cilic's serve have been great, which is why I stuck him at #10 if he can keep it up. He's played 9 matches in February, and won 75%+ of his made 1st serves in all 9, and won 80%+ in 6/9 matches while beating Murray, Tsonga, Haas, Dodig, and Rosol. But his serve improvement is just in becoming very Goran-y... his accuracy hasn't improved at all, his second serve numbers are worse, but he's hitting a huge number of aces. If he can somehow keep it up that's awesome in the strangest way; he'd be ignoring his biggest weakness (accuracy) while improving dramatically anyway.