The Hammers, having beaten the same opposition twice this season, lost the first leg of their round of 16 knockout game.
Zouma felt they could have gotten something out of the contest, especially with a last minute penalty appeal, but thought the result was fair.
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We're very disappointed," the 29-year-old told club media.
We had a lot of chances to score, but overall we didn't play very well and they did, and now we have to come from behind.
I'm not sure what was missing. We knew they were a dangerous team because we played them twice at the group stage, but sometimes you just have days like this.
Unfortunately, we didn't get the result we were looking for, but there's still everything to play for at 1-0 so we have to digest this one and try to come back stronger."