Moeen came in for criticism after taking three wickets in the 251-run Ashes defeat at Edgbaston and also making only four runs with the bat.
“I have been in this situation,” Johnson said. “I just remember my body language was very similar to Moeen, the slumped shoulders and the head down.
“Once you doubt yourself and don’t have that belief, it is pretty tough.”
In the last 12 months, Moeen has taken more wickets – 48 in 10 Tests at 25.12 – than any other bowler in Test cricket.
He was England’s leading wicket-taker in the West Indies in March with 14 but his form deteriorated and he was dropped twice during the World Cup, not playing in the latter stages.
The left-hander has made four ducks in nine Test innings in 2019 for a total of only 90 runs, 60 of which came in one innings in Antigua.
Against the Australians in the opening Ashes Test when the pitch favoured spin, he returned match figures of 3-172 in 42 overs, while counterpart Nathan Lyon helped spin Australia to victory with a haul of 6-49 from 20 overs in the second innings.
Johnson, who overcame some torrid experiences against England and their supporters to become their chief tormentor with 37 wickets in the 2013-14 Ashes series, told BBC Radio 5 Live’s Tuffers and Vaughan Cricket Show: “It’s the people that work for you.
“In that ’09 series [when Australia lost 2-1] I had a thousand people telling me what to do, all that information coming in doesn’t help so it’s going back to those people that he trusts.”
Moeen has played 11 Tests against Australia, making 476 runs with a high score of 77 and taking 20 wickets with best figures of 3-59.
“His record against Australia isn’t that great. I can feel for him a bit but that’s part of playing professional sport,” Johnson added.