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The 34-year-old, who recently had a three-year spell at Gillingham, is hoping to fire in the goals following Paul Hayes' departure, and, according to Simon, United have one hell of a player on their hands.
"His movement is superb and he has excellent vision for bringing other players into the game," said Simon.
"He has a left foot like a hammer and bags of experience. He's a great character and I expect he'll become a firm favourite with your fans if he can keep fit and stay in the side."
Of course, one weakness of the former England Under-21 international in recent years has been his injury record, but disciplinary could also be a problem.
Simon added: "His fitness record was poor at Gillingham and he never really got a sustained run in the side as a result.
"He does tend to lunge into some pretty horrific-looking tackles too, so prepare for a few bookings and maybe a red card at some stage.
"He isn't the best in the air, as we found when we continually pumped long balls up to him. Keep the ball on the deck and he'll do well for you."
Despite his undoubted credentials Johnson struggled to fully win over all of the Gillingham fans.
"He was also signed with Rod Wallace, who was a really lazy waste of space who always injured and never put the effort in," explained Simon.
"In truth, he was only at Priestfield for the final payday. Tommy was tarred with the same brush by a fair few of the Gills fans but he was a different player to Wallace.
"He DID want to play - and when he did, his effort wasn't in question, as far as I was concerned.
"He destroyed Charlton in the FA Cup two seasons ago with a superb performance and scored some fantastic goals for us during his spell with us.
"Unfortunately, we couldn't keep him fit and, on balance, he turned out to be somewhat of an expensive flop for us, which was a shame, because there's no doubt that he's still a good player.
"He has more than enough quality to do the job at League One level - keep him fit and in the side and you'll have a very influential player on your hands."
To have played for clubs the size of Aston Villa and Celtic, Johnson must have some ability, but does he still have that quality in his game?
Simon added: "He's lost a yard of pace, but is still pretty mobile - and that left foot of his is still very potent.
"If you can get 20+ games out of him, I think he'll prove to be a decent signing.
"But, with his recent fitness record, that's a pretty big 'if'."
Find out more about Tommy Johnson at Gills365.