Boss Stuart Pearce had warned Romania would be no pushovers, but second-half goals from Jordan Henderson and Smalling gave England the lead.
The visitors were not just content to sit back and a unfortunate deflection off Ryan Bertrand handed Romania an away goal.
"We knew exactly what they were going to offer, and it was a really tight game," Pearce said.
"It is totally in the balance for Tuesday night, but we are going to go to Romania and try to win another game."