Both players have struggled to hit top form this season but Rashford had netted three times in four Premier League games since Amorim replaced Erik ten Hag.
Rumours circulated on social media this week that Garnacho had been the source of leaked United team news, but Amorim said he was not sending a message to either player.
"I don't want to send a message, it is simply an evaluation and they know it. The players are really, really smart. Everybody understands my decision and I have to choose. It is simple selection," Amorim told Sky Sports.
"Of course the context is difficult because we have to win games and we have a difficult situation now. I pay attention to everything, the way you eat, the way you put your clothes to go to a game. Everything. I make my evaluation and then I decide.
"There is a communication after the last training. They are alright. They had training this morning and I was there. They trained really well."
Both sides are struggling for form coming into the derby.
Defending champions City have won just once in 10 games in all competitions to slip down to fifth in the table.
Amorim has won just once in the league since taking charge last month, but did enjoy a 4-1 thrashing of City in his penultimate game in charge of Sporting Lisbon.
United are 13th in the table, eight points behind City.
