Cundy was reacting to Monday night's 4-4 draw between United and Bournemouth at Old Trafford.
He said on talkSPORT: "They don't play in games where things are tight and boring, a bit dull.
"They tend not to play in those games and sometimes, you need to be that team. You need to have some discipline about your setup.
"To go 4-3 up, and the fourth goal that Cunha scored, it's a bit of a gift. It's a bit of a gift. You've got out of jail. You really have scored two goals in quick succession.
"Don't concede the next goal in the game. Not that they concede the next goal in the game. They concede the next two or three goals in that game. It's gung-ho.
"I don't know how United are going to be the team they want to be playing like that. I don't see it."
Cundy also warned: "If you want to be a serious team, defensively, and they zoomed in actually on Sir Alex Ferguson. And Sir Alex, they made the quote about 'attacks will win you games, but defences win you titles'.
"And it's a saying, and some people disagree, but the sentiment remains, keep a clean sheet, we'll nick a goal. And Arsenal right now, they're as good as anyone at doing that."