This could be a viable defamation claim by Trump.
That doesn't mean that O'Donnell "recklessly disregarded the truth," which is what the law requires.
First, you note that O'Donnell said "if true" and merely reported what his source told him. If that is truly the case--if there was no moment where he asserted the truth of the statement itself--that would really help O'Donnell.
The types of damages that can be alleged and proven vary from state to state.