The Colombia international threw his gloves off in frustration after being replaced by Jermain Defoe midway through the second half as Rangers moved 11 points clear in the Scottish Premiership.
Goals from Ryan Kent, Kemar Roofe, Scott Arfield and a James Tavernier penalty had Gers out of sight before the 53rd minute and Morelos was taken off with Thursday's Europa League clash against Benfica in mind. But he did not look happy to be substituted.
"Does he ever?" Gerrard said. "I can't work it out. We're 4-0 up, he's played well, his team-mate is about to come on, we've got a big game on Thursday.
"I don't know, you'll have to ask Alfredo why he was a bit sad. I wasn't sad, I was very happy."
Gerrard added: "I'm very pleased that we kept our level for 90 minutes. We could have scored even more goals, another clean sheet as well so there were a lot of positives.
"With the international break, you have players here, there and everywhere. As a manager, you are wondering whether the rhythm of your team is going to be affected but we came out of the blocks really strong.
"We played some incredible stuff again. Some of our movement in the final third was exceptional. We are showing that relentlessness and really executing when we get in the right places.
"We were strong from start to finish and I managed to make five changes and look after people who have been doing a lot of travel. As far as I'm concerned, I'm extremely happy."
Rangers extended their lead over Celtic following the champions' draw with Hibernian on Saturday.
"I look at the league table every time we play," Gerrard said. "I'm not going to deny that. But I've never known a manager to get carried away in November.
"There is a lot of football to be played, a lot of challenges, a lot of big hurdles to get over. But I won't deny I'm happy. I'd much rather be where we are of course. But stay humble."
Aberdeen were missing eight players after Connor McLennan and Lewis Ferguson were instructed to self-isolate following a positive coronavirus test for Dons and Scotland Under-21 team-mate Ross McCrorie.
Manager Derek McInnes, who was already without the injured Niall McGinn, Marley Watkins, Jonny Hayes and Dylan McGeouch, said: "We came into the game with a lot of key players missing and Scott Wright added to the issues with a groin problem.
"With the three lads out from the 21s and the injuries we had, it was a team really lacking in pace, which has been synonymous with our team this year. It was a team we felt we had to pick rather than a team we necessarily wanted to pick.
"These things are sent to try us. We tried to get a shape that would give us the opportunity to defend and have an outlet.
"I thought we settled fine. The first goal is a hell of a deflection and the second goal we can do better. It comes probably in our best spell, we have threatened.
"Half-time we spoke about trying to get the next goal and six minutes later we are out of it after another deflection and a soft penalty, the big boy (Leon Balogun) went down pretty easily.
"It didn't feel as though we got the breaks, my players worked extremely hard despite the obvious difficulties. There was plenty effort from a lot of players who haven't played a lot of games of late."