The Belgian felt his side had played a strong match on Monday but let their organisation slip at crucial moments and got punished as a result in their opening match in Libreville.
"We tried to play quicker in the first half, chasing the play from one side to the other and with lots of tempo, which is our normal game," said Gerets.
"Collectively I think we played pretty well. It was good football and I think maybe we were unfortunate not to score more.
"But we let the heart overrule the brain at key moments of the match. There were eight minutes in the game when we lost concentration and organisation. It should be a good lesson to the players that even if you are attacking you need to keep your organisation at the back."
Morocco were stung by a freaky Tunisian goal in the first half as the ball floated in from an innocuous-looking free-kick and then by a swift counter-attack in the second half. Morocco only pulled one goal back with six minutes left and ran out of time in search of an equaliser.
"This result puts a lot of pressure on us, there is no illusion about the fact we must now beat Gabon in our next game," Gerets added.
Morocco's next Group D game is against the co-hosts in the capital Libreville on Friday.
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