是的，greatly 可以修饰介词短语，举例如下：

I was greatly in awe of Jane at first. 起初我对简很敬畏。

They are greatly in need of our help. 他们迫切需要我们帮助。

His shabby appearance is greatly against him. 他褴褛的外表对他很不利。

Greatly to his credit, he came out first. 他非常光荣，得了第一名。

Greatly to my regret, I cannot attend the party. 很可惜的是，我无法参加聚会。