Joao Cancelo played 45 minutes in the goalless draw in Germany before he was withdrawn at half-time.
Cancelo has again been one of Guardiola‘s most used players this season but has suffered with fever since Saturday’s Premier League victory over Brighton.
With Kyle Walker injured, Sergio Gomez suspended and John Stones still working towards full fitness following his return from injury, Guardiola felt he had to select the full-back on Tuesday night.
“Today we were exhausted, the game against Brighton drained us a lot - it was demanding, man to man, it was physically so demanding,” he said.
Dortmund 0-0 City | Match stats and reaction
“Joao with fever made an incredible effort. I want to say thank you to Joao because we don’t have Sergio or Kyle and John made an effort to come back.
“I appreciate what they have done and Julian made an incredible effort defensively. It was so difficult for him but in general, especially in the second half, we did a good performance.”
Erling Haaland was also removed at half-time, and Guardiola explained why in his post-match press conference.
“Three things: I saw him so tired,” he started.
“Second one, he had a little bit of influenza in his body. Like Joao - Joao had a fever.
“Then the third, he had a knock in his foot. That’s why he was not able to play the second half.”
