Romo won't know a timetable for recovery until more tests are taken Tuesday. Cowboys owner Jerry Jones said he was told 6 to 8 weeks, although he noted that Hall of Fame quarterback Troy Aikman once missed just four weeks with a similar injury.
"It'll be tough," said Romo, who has missed just three games since replacing Drew Bledsoe as Dallas' quarterback in October 2006. "The train keeps going on. The games are going to be played."
Romo was drilled by blitzing Giants linebacker Michael Boley with 12:07 left in the second quarter Monday night. It was a clean hit, but certainly a big shot -- Boley was untouched, and Romo was vulnerable after having thrown a pass. The quarterback went down hard on his left shoulder and remained flat on his back.
"The guard didn't see me," Boley said. "I came in scot-free. ...
"When he (Romo) hit the ground, I heard him let out a little scream. I knew something was up, but I didn't think he was going to lay down. I thought it was a normal hit. After I got up and started running, I looked back and saw he was down."
Romo learned of the break before halftime, but he returned to the Cowboys' bench at the start of the second half, his left arm in a sling and covered by a jacket. He also wore a headset to stay in tune with the game.
When Dallas' first series fizzled, Romo and his replacement, Jon Kitna, discussed what went wrong, and looked over pictures of the plays with third-stringer Stephen McGee. Romo then stood alone and watched the offense go back for its second try.
Romo took every snap last season -- making Kitna the NFL's only No. 2 quarterback not to play -- and had done so through five games this season.
Romo had thrown every pass by a Cowboys quarterback since Nov. 16, 2008, when he returned from a broken pinkie on his throwing hand that cost him three games. He set the franchise record for passing yards in a season last year, making the Pro Bowl for the third time.
In his brief action Monday night, Romo broke his own team record by throwing for a touchdown in his 18th consecutive game. He was 5-of-7 passing for 39 yards.
The Associated Press contributed to this report.