The Daily Mirror reports that Will wasn't keen on the signing because of his friendship with Gavin's ex-wife - and coach on The Voice in the US - Gwen Stefani.
Gwen and Will have toured together and the Black Eyed Peas singer supposedly wasn't impressed by her ex-husband sitting alongside him on The Voice here in the UK.
Will had signed up for The Voice UK back in July, having appeared on the BBC One version of the show, but Gavin joined the cast closer to filming beginning in October.
A source spilled to the Daily Mirror newspaper: “ITV had to pull out all the stops to try and persuade him to stay on the show.
"Will felt he was placed in a really difficult situation because of his friendship with Gwen."
According to the insider, Will "wasn’t happy at all" with the situation but was eventually "talked around", although it was apparently "touch and go".
The source explained: "He was eventually talked around not least as he is such a professional.
"He has put any issues he might have had aside and is determined to make this series of the Voice the best he can.”
There were certainly no signs of problems on the panel last Saturday night when The Voice UK kicked off its first ever series on ITV.
Alongside Gavin and Will, the channel secured Jennifer Hudson and Sir Tom Jones for this year's show.
A spokesperson for The Voice UK said: “There is no animosity amongst the coaches and they all speak highly of one another.”
The Voice UK 2017 continues Saturday night at 8PM on ITV.