The former Aston Villa man struggled to adapt in the early stages of his time at Anfield and was publicly questioned by manager Brendan Rodgers earlier this season.
However, Downing, who signed for £20m in 2011, has shown an upturn in form this year and Johnson has spoken of how highly he rates his England colleague.
"Stewart did knuckle down, put his neck on the line and went for it. It's great to see that now he's getting the rewards and credit he deserves," Johnson told reporters.
"He has been fantastic; his work-rate, the way he gets at players and defences. A lot of the time in football, a lot of hard work can go unnoticed.
"I've been a big fan of his for many years. We play well together and cause other teams quite a lot of problems. He's been brilliant, I've always enjoyed playing with Stewart."
Both Johnson and Downing have started 10 of the last 11 games together for the Reds.