Roberto Mancini didn't make any halftime changes to his Manchester City side, but his patience apparently wore out around the 50th minute. With Adam Johnson unavailable, Mancini has (hopefully) spiced up this edition of the Manchester Derby against Manchester United by bringing on Shawn Wright-Phillips for Aleksandar Kolarov.
So, everyone's progression of thinking. Almost everyone's first reaction was "SWP?!?" before they realized that Adam Johnson is injured and it's really not all that stupid. Then, everyone saw the fourth official's fancy flashing light board and went "Kolarov?" Pablo Zabaleta would seem to be the more logical switch, after having a poor first half. Kolarov could easily move from left midfield to left back, since he plays the latter position much more often. This is not to say Kolorov absolutely should not have been subbed - he's been average - but he's certainly been better than Zabaleta.
As a result of the substitution, David Silva is now playing much more on the left. Previously, he was a central player with freedom to roam around. This should mark a fairly significant tactical change.