Speaking after Barcelona's 2-1 victory at Camp Nou, Armstrong and Mendieta agreed that Madrid boss Carlo Ancelotti made a serious selection error when deciding to play centre-back Sergio Ramos in a central midfield role.
Goals from South American duo Neymar and Alexis Sanchez gave Barcelona victory in the first meeting between the two La Liga rivals this season and first-blood to new Barca boss Tata Martino.
Former Mallorca striker Armstrong was confused with Ancelotti's line-up and questioned his tactics.
He said: "It was a negative start from Ancelotti's selection - Sergio Ramos in the centre of midfield as an anchor-man, what was he thinking?
"I think he must have thought he was going to put pressure on the midfield and put pressure on Messi when he was picking up the ball and turning and running but he was out-thought because Tata Martino had Messi playing on the right-hand side. Ramos was always on the wrong side for me. He never really seemed to be in the right position, he seemed lost.
"As a centre-half he's very decisive. He makes big decisions and is very positive but because he was playing front of the two centre-backs he didn't seem to be getting involved in the game and it sort of passed him by," added Armstrong.
"He was also so fortunate to not be sent off. He's an impetuous player and his record of sending-offs and yellow cards shows that. He's played as a right-back and as a centre-back, I've never seen him play in midfield - that was a big gamble and I think it cost Real Madrid big time."
Former Barcelona midfielder Mendieta agreed with Armstrong and was puzzled as to why Ancelotti did not select a more suited player than Ramos, such as Pepe.
He said: "This was a big gamble play in him in this position for such a big game.
"If Ancelotti was thinking of having a physically strong presence in the midfield to stop Messi in midfield, why not put Pepe there? - he's played more times in midfield than Ramos - I don't think he's ever even played there and you could tell that because for most parts of the game he was just jogging around and never really closing down or putting pressure on the ball.
"He was just more worried about holding his position in front of the two centre backs."