Manchester City closed the gap to Premier League leaders Arsenal back to five points and left Chelsea languishing down in 10th position after a 1-0 win at Stamford Bridge on Thursday.
After City struggled to make inroads in an unfamiliar formation in the first half, substitutes Jack Grealish and Riyad Mahrez combined on 63 minutes for the Algeria winger to tap home and grab a vital three points.
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Both teams had earlier hit the woodwork but it was Guardiola's changes, in personnel and abandoning a three-at-the-back starting formation, that proved decisive.