Antony, on-loan at Real Betis from United, struck as the LaLiga outfit defeated Fiorentina 2-1 in the first-leg of their Europa Conference League semifinal.
Afterwards, De Gea conceded: "I've trained with him a thousand times and he hasn't scored a single goal with his right foot in any training session.
"He's in very good shape, he has a brutal one-on-one style, with confidence; when a player is confident, it shows how good he is.
"He had a great game and a great goal. I didn't see it come out , there was a player in front of him and I couldn't see it.
"He hits it very well with his right foot and I'm happy for him because he had a difficult time in Manchester and he's showing what a good player he is."