Leeds had failed, along with Wigan, in a previous approach for Senior, but his arrival now is a boost to their hopes of going one better than last season, when they were beaten in the Grand Final by Wigan.
"It was too good an offer to refuse," said Sheffield's chief executive, Ralph Rimmer. "Keith had been unsettled for some time and it is a good deal for all three parties."
Senior called it "exciting to be coming into a squad with the likes of Iestyn Harris and Adrian Morley. I have to prove my worth to Graham Murray and hope I can be part of the play-offs with the team."
There is an old Yorkshire litany that pleads: "From Hull and Huddersfield and Hell, Deliver Us". In the absence of a franchise application from the underworld it will be two of the game's most famous clubs seeking deliverance tomorrow night. If Huddersfield beat Castleford, they will stay up and Hull will drop into the flames; if they lose, Hull can survive by beating Sheffield.
Hull have potentially crucial advantages. Badly run as the club has been, it retains the fervour of its support, if not the sheer numbers. That could be crucial against a Sheffield side which has little to play for and could be without Darren Turner and Darren Shaw as well as Senior and the suspended Dale Laughton.
Hull will be without their most consistent forward, Steve Craven, whose cheekbone fracture led to London's Dominic Peters being suspended for seven months this week.
Like Hull, Huddersfield can point to times when they have been the dominant team in the code. The town is the birthplace of rugby league, but that was long ago and, in the two years since promotion, they have shown even less sign than Hull of adapting to life in Super League.
Their hopes against Castleford depend largely on whether last-minute signings such as Dean Lawford, Gene Ngamu and Troy Slattery can perform to the standard required.
Their situation is complicated by the fact that Cas are still not mathematically certain of their place in the top five. Defeat at the McAlpine Stadium and a high-scoring victory for Gateshead at Warrington could deprive them of what they have worked for all season; it is unlikely, but real enough to keep them honest during this week's build-up - and that bodes ill for Huddersfield.
The former Castleford and Bradford player, Graeme Bradley, has been banned from taking any part in the game for 12 months. Bradley, who was then coaching Oldham's Alliance team, refused to leave after being sent off at Leigh.Reuse content