Demba Ba scored the only goal in the 11th minute to send the Blues through along with Schalke.
However, Terry felt Chelsea had a lot of room to get better and told Sky Sports 3: "It was okay.
"We are still improving and can get better for sure.
"It was important for us to top the group tonight, some tough teams have topped the groups."
The former England skipper pointed to Chelsea's inability to kill off games, saying: "It is something we need to work on.
"It is important in games like this home and away we kill off the game."
Of their prospects now, Terry said: "We'll go about our business, we are through now, to top it is a bonus.
"We are quietly confident, we know we have to get better and improve."
The Blues took a 10th-minute lead due to Steaua's shaky defence.
Initially it was deemed to be an own goal by Daniel Georgievski, who also found his own net in Bucharest, before Ba was subsequently confirmed as the scorer.
Of whether he did get the touch, Ba said: "I think so, I hope so.
"At the end of the game I was really tired, I need to work on my fitness."