The Spanish boss admitted he was anxious when the 26-year-old German, making his first start in nine months, had to be substituted just before the hour following a challenge from Claudio Yacob, per Paul Hirst in The Times:
"I think it's a little injury. Unfortunately the action was tough but he will be back soon hopefully. He will have a scan, but the doctor told me it's a little [injury], and not a big issue.
"I suffered for Gundogan in that moment when he was on the ground. At that moment, you think of the wrong situations, you cannot imagine what it is like to be out for months fighting every day alone with the physio and come back and to be injured. In that moment, it's so tough to accept. It's football and fortunately it's not serious."
City beat West Brom 2-1 thanks to a Leroy Sane double and have been drawn against Wolverhampton Wanderers in the fourth round.
Gundogan's fitness, though, was the main story of the night at the Hawthorns given he had not been included in a City starting XI since December 14, 2016, a 2-0 win against Watford during which he suffered a cruciate knee ligament injury.
Before he was felled from behind by Yacob soon after half-time, the former Borussia Dortmund star showed just what City had been missing in 2017, per ESPN's Jonathan Smith:
Jonathan Smith @jonnysmiffy
For his first start in nine months, Gundogan has been absolutely fantastic. Running the game. City should have this game over and done with.2017-9-20 19:42:27
When Gundogan went down it did not look good, per the Manchester Evening News' James Robson:
James Robson @JamesRobsonMEN
That did not look good for Gundogan. Looks like he's hurt his left ankle after Yacob challenge from behind2017-9-20 20:13:07
However, Guardiola's assurances that his latest knock is not serious will be a huge relief to all involved at City.