John Terry's substitution against Benfica was a bit of a headscratcher - the captain was playing very well, making up for David Luiz's relatively poor performance with a wonderful, assured display. Gary Cahill is a decent enough player but he wasn't up to the task in this match. Without Terry, Chelsea collapsed, and were perhaps lucky to advance to the semifinals thanks to Raul Meireles' very late intervention.
So, why was Terry pulled? Turns out the reason isn't that Roberto di Matteo has gone insane.
Yikes. Hopefully this turns out like the David Luiz ankle injury* and he bounces back fine for the next match, but Terry's leadership is going to be vital for Chelsea's hopes of an FA Cup win and finishing in the top four. If Terry's actually out for a substantial amount of time, we're in deep trouble. Fingers crossed those scans come back with happy news.
*Given his performance today, I'd very much doubt he was 100% fit.