According to the Daily Mirror, De Bruyne sustained a knee injury on Wednesday in training during an outdoor session.
City said the extent of the injury suffered by the 27-year-old Belgium midfielder is "not yet known".
The news had an immediate impact on the Exchange victor market, with City drifting from [1.6] to [1.7] and second favourites Liverpool backed in from [5.0] to [4.3].
Nevertheless, Gundogan is hopeful De Bruyne is able to feature again soon given his importance to their title charge last term.
Cincinnati Masters: Novak Djokovic overcomes illness to see off Mannarino
The Dutchman, ranked 55th in the world, advanced at the expense of world number four Zverev by defeating the German 5-7 6-4 7-5. Alexander Zverev crashed out of the Cincinnati Masters after a three-set defeat to unseeded Robin Haase in the second round.
"I hang out a lot with David De Gea because we play together at Atletico Madrid", the Argentinian striker says.
Kyle Walker is also confident City have the depth in their squad to deal with the potential long-term absence of playmaker De Bruyne.
"At 1-0 we found a goal maybe we didn't deserve but in the first half we deserved to score two or three more".
Scarlett Johansson leaps to top of Forbes' highest-paid actresses
Forbes said the lack of roles for women means "there are fewer opportunities for stars to earn big bucks". Currently, female characters fill only 28.7% of all speaking roles in the film, according to a 2016 study.
"In the beginning it was a bit weird as you don't know the people you are working with but then after time you forget they are there".
"You killed them while playing football", Guardiola said. "It is a different prospect, cup games are different to a league".
We've already seen the spine-tingling trailer and the clip of Pep Guardiola berating his players during half-time of their FA Cup defeat to Wigan last season.
The 47-year-old former Barcelona and Bayern Munich boss continues to win plenty of plaudits, but he claims there are chinks in his armour which he tries his best to cover up.
Aretha Franklin Tribute: Ariana Grande Performs 'Natural Woman' on Fallon
Also, "Another knowledgeable source described her as "despising" everything he stood for, as an avowed Hillary Clinton supporter". Also, he harshly said, "The Queen of Soul, Aretha Franklin , is dead" on Twitter . - Franklin has won 18 Grammy awards .