It led to fan speculation as to whether he was injured or still suspended following last season's red card against Villarreal in the penultimate group stage match.
Gerrard insists it's simply a case of him having a large squad to choose from and the need to rotate for a long season ahead.
He also told the rest of his team that they'll have to take such disappointments on the chin and be determined to win their place back.
He told the Scottish Sun: “Daniel isn’t suspended or injured, it’s just a numbers game.
"Daniel has been left out of this squad, but he won’t be the first, and he won’t be the last, big name to be left out.
“I’m not going to make a big thing of it. It’s just the way it is. We have big squads and big players, and competition for places.
"It’s normal from week to week that there are changes, who is in and who is out. Daniel is a good player, he played a lot of games for me last year.
“The same could happen again this year. I wouldn’t be surprised if he is involved in the competition in the next round if we were to progress."
He added: “With any players, if they are not in the 11 or the 18, they are within their rights to feel disappointed.
“But you also expect a level of professionalism where you get on with it, and you fight for the jersey and fight for your place.
“You take it in the right way and take it as a challenge to win the shirt back."