"We were pretty comfortable first half. The second half, the manager (Jose Mourinho) kind of stressed that we can't get a little bit sloppy and throw it away, and that's exactly what he predicted," Terry told Sky Sports News.
"We've been killing teams off in the Premier League, but tonight, we didn't do that and we're disappointed to lose a goal on the counter.
"We've got some tough games coming up and I think in the second half, we were very slow."
Elsewhere in the group, Maribor drew 1-1 with Sporting Lisbon.