The decision was questioned by the City Ground's support.
Afterwards, Pereira said: “Morgan is a fantastic player. But in the moment of the subs, usually I give some minutes to understand if the player has the energy to help the team and be himself.
“He ran a lot in the last game (against Midtjylland). For me, Morgan is not a winger. He is a player to play in between the lines and to play in the back of the striker.
“At half-time, I realised I didn’t want him to play as a winger because I prefer to bring him into the areas where he can decide the game. He tries.
“I put a true winger on to create situations one-vs-one. Omari (Hutchinson) did very well.
“I tried with the substitution of Igor to put more energy on the pitch, with Taiwo. Taiwo created a lot of problems.
“I tried with Dan (Ndoye) to have another fresh winger. He likes to attack the space. For me, that was important in that moment. We tried everything the game was demanding, even the substitution of Morgan.
“(James) McAtee is a player that, if he plays with confidence and what he does in training, he can do magical things. From nothing, he can create something special. In the last minute, I thought we would try with this kind of player because sometimes you can get a penalty or sometimes a situation one-vs-one. He has the quality to do it.
“It is not easy in five minutes, of course. He tried. I asked him to go and play with confidence. I wanted to see the same player I watch in training. We tried everything.”