Chelsea boss Jose Mourinho says extra time would have been a problem had Jermaine Grandison not scored an own goal in the final minutes of the game.
In-form striker Didier Drogba gave Chelsea the lead against a determined Shrewsbury side but Andy Mangan restored parity in the 77th minute of the game before Grandison's own goal saw the Blues reach quarter-finals of the competition.
"I didn't change my feelings [that] the game was going to be difficult," Mourinho told Sky Sports.
"It was difficult from minute one to the last minute. I still had [Eden] Hazard to make a change but I was worried about the possibility of going to extra-time.
"We needed to do that change so I delay that change, could be a change to break the game definitely but when you have the possibility of extra-time you have to control the situation which he did so I'm very pleased with the players, I was not expecting a different match."
A number of players were given a rare chance to shine for the Blues but Mourinho was not amused by the performance of some.
"I expect people that have not been playing a lot to raise their level and create problems," the manager is quoted as saying by the BBC.
"I love problems of choice, but it's easy to choose my team for Saturday.
"I also had people like [goalkeeper] Petr Cech and [midfielder Jon Obi] Mikel, people with lots of years at the club, who would prefer to start at Old Trafford.
"They didn't and they came here today and gave a fantastic example. After that there are the young boys and they have to follow the spirit and the professionalism of the others."