The New Zealander was preferred to former Saints and Great Britain star Leon Pryce at stand-off having impressed off the bench in Sunday’s Magic Weekend win over Hull KR.
That was only Tuimavave’s first game back after six weeks out but Radford gave him the start at KC Stadium today and he came up with two crucial plays as Hull reclaimed Super League’s top spot.
They had been cruising at 12-0 up early on and, with Saints losing chief playmaker Luke Walsh to injury inside two minutes, seemed destined for an 11th win in 12 games.
However, the visitors turned things around with three tries in 15 minutes to lead 18-14 at the break.
But Tuimavave, the rangy Kiwi in his first season after joining from Newcastle Knights, dummied his way over in the 56th minute to reclaim control and then produced the kick that set up the excellent Scott Taylor’s try, too.
Marc Sneyd, who kicked six goals from as many attempts, converted both but - after Jack Ashworth hit back again for stubborn Saints - it was not until Danny Washbrook claimed a drop-out and sprinted 20m in the 77th minute that Hull could truly relax.
They had won 47-18 at Saints in the Challenge Cup earlier this month but this fierce contest was more akin to the 17-16 league victory there on April 1.
Lee Radford’s side utterly out-played Saints in all departments.
Liam Watts charged through a gap to set up captain Gareth Ellis between the posts, and when Jack Owens spilled on his own 20m line, they were in again.
This time Sneyd’s fine pass sent Mahe Fonua over untouched as Saints’ defence disappeared again.
Sneyd converted both and with props Watts and Taylor off-loading at will, Hull were too hot for sorry Saints.
When the hapless Owens spilled again, Theo Fages, crucially, denied the hosts a third try when he denied Jamie Shaul just as the full-back latched onto Sneyd’s pinpoint grubber early in the tackle count.
That was a turning point as if the scored had extended to 18-0 there would have been no way back.
Instead, Lomax fired up the rally to get Hull jittery. Saints finally got some field position with a penalty and his cut-out pass put Adam Swift in at the corner.
In the next set, the exciting full-back sliced through some flimsy defence himself for Owens’ second conversion to level at 12-12.
But there was still more as rattled Hull stumbled again. Jon Wilkin burst through Fonua’s tackle at a scrum to surge 50m and, though he was hauled in by Shaul, the danger wasn’t over.
Fages threw a pass that hit the floor - and seemed to easily go forward - but referee Gareth Hewer waved play-on, Sneyd could not gather and, instead. Atelea Vea picked up for an easy try in the 37th minute.
Owens improved as Saints claimed an 18-12 lead narrowed slightly by Sneyd’s penalty after the half-time hooter.
Hull second-row Dean Hadley looked certain to score but was denied by desperate Saints defence and the visitors would not subdue.
They even pushed at a Hull scrum and won the ball back only for Greg Richards to fumble as he tried stretching over.
Instead, after Jordan Thompson made a searing break to clear their lines, Tuimavave came up with that telling try - only his third in Black and White colours.
Next his chip saw Saints prop Kyle Amor panic behind his own line, gifting Taylor his fifth try in as many games in the 62nd minute.
Ashworth gave Saints hope but Washbrook made sure with his first try in 15 games since re-joining from Wakefield Trinity.
Hull FC: Shaul; Naughton, Fonua, Yeaman, Talanoa; Tuimavave, Sneyd; Taylor, Houghton, Watts, Minichiello, Manu, Ellis. Substitutes: Thompson, Hadley, Bowden, Washbrook.
St Helens: Lomax; Owens, Dawson, Turner, Swift, Fages, Walsh; McCarthy-Scarsbrook, Roby, Amor, Wilkin, Vea, Knowles. Substitutes: Walmsley, Tasi, Richards, Ashworth.
Referee: Gareth Hewer (Whitehaven)