D'Arnaud suffered what initially was labeled a hyperextended elbow when he applied a tag on Atlanta's A.J. Pierzynski on a play at the plate on Saturday at Turner Field.
X-rays taken in Atlanta did not reveal a fracture and d'Arnaud was labeled day-to-day at the time.
With d'Arnaud having not improved during Monday's off-day, the Mets dispatched him for an MRI in Milwaukee. Team doctors then requested to examine d'Arnaud in New York on Tuesday, assistant GM John Ricco said.
"I just know the doctor said he's got a sprain a little bit, so he's going to need to probably go on the DL," manager Terry Collins said.
Rookie Kevin Plawecki should handle the bulk of the catching duties in d'Arnaud's absence.
The Mets promoted catcher Johnny Monell from Triple-A Las Vegas to take the roster spot.
Team officials may have been inclined to promote fellow catcher Anthony Recker from the Pacific Coast League club instead of Monell. However, Recker had not spent the required 10 days in the minors and was ineligible unless a player simultaneously went on the DL. And with the Mets initially unsure d'Arnaud would land on the DL when they made the promotion decision, they opted for Monell. The decision may be revisited in a couple of days, Collins said.
The oft-injured d'Arnaud had just returned from a nearly two-month DL stint on June 10. He had suffered a fractured right pinkie on April 19 when he was struck by a fastball from Marlins reliever A.J. Ramos. D'Arnaud then was slowed during his minor league rehab assignment because of a bone bruise in his right wrist.
D'Arnaud spent time last season on the DL with a concussion. In 2013, he missed nearly the entire first half of the season with a fractured left foot.
"You know what? He gets hurt. And when he gets hurt, it's legit," Collins said. "It's not like this guy's got a bump and he won't take two aspirin to come back the next day. This guy, it's legit. We'll just wait the two weeks and hopefully get him back and we'll continue to move forward."
"We're pretty much numb to the whole situation that every time we turn around something happens," Collins said. "We've just got to regroup. It doesn't matter who is in there, we've got to get some production out of some guys."