The Three Lions currently trail the five-match sequence 1-0 following a defeat at Old Trafford that came in between washouts at Headingley and Edgbaston.
Losing in Cardiff on Saturday would result in a series win for Australia, and Morgan has told his team to be wary of the threat posed by Johnson.
"I think he's bowling all right at the moment," Morgan told reporters. "He bowled well in the last game, and we know his capabilities. We've played against him for years.
"We know when he bowls well, he's pretty dangerous. We're aware of the fact he's a wicket taker and probably the guy who leads their attack."
Johnson impressively dismissed Kevin Pietersen on Wednesday.