The plan was originally to wait until Wednesday to make the roster move, but Red Sox manager John Farrell wanted to be sure he had a fully staffed bullpen, especially in the event of inclement weather.
"Given the weather tonight, we can't afford to get in a stop-and-start situation, which could very well take place tonight," he said.
For Morales, this will be his second time on the DL this season with both issues stemming from problems in his pectoral muscle. The lefty felt discomfort in his throwing shoulder during his last outing Saturday against Detroit and after being examined by team doctors Monday, he was diagnosed with a strained left pectoral muscle.
"He's got a strain to the left pec," Farrell said. "Not the exact same spot that he dealt with in Spring Training, but not too far away from it. Not a severe strain, but one nonetheless. It's clear. The images show it. So how long the down time is yet to be determined at this point."
In six appearances this season Morales accumulated a 7.30 ERA in 12 1/3 innings.
In two previous appearances this season, Beato has pitched two scoreless innings.
Michael Periatt is an associate reporter for MLB.com.