The 25-year-old hopes to find the net for the third successive game against Charlton Athletic today.
Prior to that he went 11 games without scoring - but Ferguson never lost sight of the former Millwall frontman’s value to the team.
“He was always in a good frame of mind, we had regular conversations,” said Ferguson.
“There was never any doubt in my mind that he would get back to scoring goals.
“For me he’s one of the best strikers in the league, there’s no doubt in my mind.
“I know he perhaps doesn’t score as many goals but ask any defender that plays against him - they know they’ve been in a game.
“I think he’s done tremendously well and he’s kept his belief in his performances and not let it affect him.
“When he wasn’t scoring he was having a big impact on the team.
“There’s players you just can’t afford to lose and John’s one of those.”
Marquis has scored nine goals this season.
Will Longbottom has joined Evo-Stik Premier Division side Halesowen Town on a one-month youth loan.