The ECB is expected to start a quantitative easing program it hopes will provide a boost to the European economy.
"We're going to end up seeing that Mr. Draghi will not be able to do what the market really wants him to do. He needs to get the balance sheet of the ECB back to $3 trillion, where it was several years ago. The problem that he has is that he doesn't have the ammunition or he doesn't have the capability to get it there," the editor and publisher of The Gartman Letter said in an interview with "Closing Bell."
While the U.S. has broad federal debt securities, the ECB has 19 different treasury securities from which to buy.
"He would like to get it done. Size counts. Size matters. But I'm not sure he can get the size accomplished. So it will probably be a bit of a disappointment but at least let's say he'll give it a good college try," Gartman said.
The markets are anticipating about 500 billion euros ($580 billion) in bond purchases, but some economists think it could be higher. On Wednesday, sources confirmed to CNBC that the central bank is planning to announce it will purchase 50 billion euros of bonds a month. The Wall Street Journal first reported that figure.
"Let's give him credit for being able to accomplish anything. This is a very tendentious group of people, of countries, that he has to try to get together," Gartman said.
Whatever Draghi can get done will help the European economy, he said, and will also put downward pressure upon the euro two to three weeks from now.
"But you're likely to get a small bounce. Any bounce that you get on the euro predicated upon disappointment ... in tomorrow's action ... should be sold into," Gartman added.