Scotland hero McFadden was helpless to prevent Well from crashing to a 1-0 loss to United on his return to action at Fir Park last night.
The disappointing result meant that Stuart McCall's team stayed just two points ahead of Inverness Caley Thistle in the SPL table.
However, winger Humphrey still believes the capture of the former Everton and Birmingham City forward will be massive for the Steelmen.
He enthused: "James is a great guy. He has come in and slotted right into things. It is good that something has been sorted out with him. He will strengthen our squad massively.
"You could tell the level he has been playing at straight away in training. His touch, his awareness and his ability are plain for everyone to see. It is good for a guy to train like that.
"He will bring a wealth of experience given the clubs he has been at. I think he will bring leadership qualities as well. He will definitely enhance our chances of finishing second in the league."
Humphrey added: "Even if we hadn't signed him I think we could have finished second. We have shown this season that we are more than capable of finishing second without him.
"Yes, it was a disappointing performance and result against United, but we just have to try and put it behind us and get back to winning ways.
"Hopefully we can carry on playing the way we have been playing and avoid picking up any more injuries. We have more than enough in the squad now to finish second."
Johnny Russell put United in front in the 17th minute after running onto a through ball from Gary Mackay-Steven and slotting the ball past Darren Randolph.
That strike was very much against the run of play as the home team had dominated the opening exchanges and looked far more likely to score.
However, Jackie McNamara's team nearly added to their tally shortly afterwards when a long-range Russell shot went narrowly wide.
Motherwell's hopes of getting back into the match were not helped early in the second half when their captain Stevie Hammell hobbled off injured.
The left-back was replaced by Zaine Francis-Angol and the teenage defender worked hard to help create the opening McCall's men craved.
THE Steelmen had Randolph to thank for keeping them in the match in the 65th minute when he denied a Mackay-Steven shot with his legs.
For all their possession and pressure, Motherwell created precious few real goalscoring opportunities against a team seven places below them in the table.
Stuart Carswell at least had an attempt on target in the 71st minute from the edge of the box – but it was easily gathered by United keeper Radoslaw Cierzniak.
His opposite number produced fine blocks in quick succession from Mackay- Steven, Russell and Stuart Armstrong as the Tannadice club grew in confidence.
Michael Higdon had his appeals for a spot-kick waved away by referee Stephen Finnie after Jon Daly appeared to handle his shot inside the penalty area.
McFadden, who was released by Barclays Premier League club Sunderland last month, was hardly involved after replacing Kallum Higginbotham in the 73rd minute.
The man who launched his career at Motherwell had one lively run at the opposition goal – but he was dispossessed by the United defence and it amounted to nothing.
It would have been a travesty of justice if Dundee United had failed to pick up all three points as, towards the end, they pummelled their opponents.
The Tangerines are now just four points off second spot themselves and, on this evidence, they must have a decent chance of finishing in second place in the Premier League themselves.