David Murphy gave Birmingham the lead in the FA Cup fifth-round tie after 20 minutes, and Villas-Boas opted to replace Torres with Didier Drogba for the second half as the Londoners searched for an equaliser.
But it was England striker Daniel Sturridge who netted the all-important equaliser for the Premier League side in the 62nd minute to set up a replay at St Andrew’s.
“It is a poor result of course, 1-1 is not what we expected, but it gives us a chance to go to Birmingham and try and win and get the trophy,” Villas-Boas told ESPN.
“We needed more of a target man up front, laying off the balls, so we made a change. I’m not concerned with Torres’ physical form, he’s looking for the goals gives him extra motivation, extra stimulus.
“The desire was there, and the ambition, they matched us in that sense, their goal gave them further motivation, to get the draw and go back to their ground.”
Chelsea struggled in the absence of John Terry, and poor defending from a Keith Fahey corner allowed Murphy to volley home the opener.
Villas-Boas’ side were immediately handed the opportunity to restore parity when referee Martin Atkinson penalised Wade Elliott for bringing down Ramires in the area.
However, Birmingham goalkeeper Colin Doyle saved splendidly from Juan Mata to keep the visitors’ slender lead intact.
The introductions of Drogba, Frank Lampard and Salomon Kalou after the interval helped the Blues improve on a dire first-half display.
And Sturridge rescued the home side from a cup upset when he headed home Branislav Ivanovic’s cross to set up a replay.
“We’re disappointed with the result, we’re playing at home, we wanted to get the victory and go onto the Champions League,” said Sturridge.
“It was good from a personal note to get a goal, we were hoping that would push us on, but we couldn’t get another one.
“Any team at home wouldn’t want a draw against a team in the league below, but we played great football today.”
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