The Hoops were blown away this week by the Bundesliga high-fliers and now face a near-impossible task to overturn the first leg hammering on Thursday.
Moroccan international El Khannouss helped himself to a first half double and the bad news for Celtic is that he believes they will improve this week.
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He said: “We know we can still play better than we did in Glasgow.
“I scored two goals, but there were also aspects of my performance that I wasn’t happy with and could have done more.
“But of course I cannot complain after scoring two goals.
“I have put in a lot of effort recently. I had a gym installed in the basement of my apartment, where I work every day on my physical condition and that’s helped.
“I think the extra sessions paid off in Glasgow.
“We got the win, but we have to concentrate on being professional in the second leg and completing the job.”