The highly-rated England Under-21 international made a loan move to Vicarage Road in 2007 and it was during his spell with the Hornets that he made a name for himself as he scored five times in 12 games.
While Johnson admits he is indebted to Watford for furthering his career, he is adamant he will not be in a sentimental mood when they travel to the Riverside this weekend.
"Going there was the best decision I ever made," Johnson told the Middlesbrough Evening Gazette. "It was fantastic for me - probably the most important spell in my career.
"I had come on so much in that spell there and ideally I would have stayed a bit longer and played more games.
"It's still the first result I look for. There's been a lot of changes but I still have a lot of friends down there - and I'll be out to put one over on them and win on Saturday."