Millington scored the opening try just six minutes into his comeback from a knee injury suffered in the Challenge Cup quarter-final win over Salford two months ago.
It was his only chance to secure a place in the Wembley side to face St Helens on Saturday, after he missed out through injury when Tigers lost to Leeds Rhinos in the 2014 Cup final.
Coach Daryl Powell, who rested most of his first-choice players, said: “I thought he was good.
“He was always going to be a bit rusty, but he is an important part of the squad and team and he has put himself right in there for selection.”
Millington’s try and the first of three conversions by Danny Richardson gave Tigers a 6-0 lead, but Salford were 22-6 ahead at half-time.
Jack Ormondroyd got them on the board and Ollie Roberts, Morgan Escare and Rhyse Williams all crossed as Tigers, including five teenage debutants, struggled to handle the visitors’ size and pace.
It was a similar story after the break.
Brad Martin got Castleford off to a flying start, but Salford responded quickly through Ken Sio and then the floodgates opened as Escare, Dec Patton and Matt Costello took the visitors past the 40-point barrier.
Chris Atkin brought up the half-century with 15 minutes left before Alex Foster went over for the hosts’ third touchdown, but Salford added three tries in the final minutes, through debutants Dan Norman and Ben Davies, plus Williams’ second. Krisnan Inu missed only one of his 12 conversion attempts.
“We gave two points up,” admotted Powell of his side’s fifth successive home defeat.
“But the upside was five players got an opportunity they wouldn’t normally have got and they will have learned a lot.”
He added: “This season has thrown up a really crazy fixture list and there’s no way I am going to put most of our players for the Challenge Cup final into a game six days before.
“It doesn’t make any sense, so I made that decision and I thought the young players were great.
“They found it tough, which is obvious, but we either had young players out there or players who haven’t played for one, two or three months.
“It was difficult at times and Salford scored a lot of points, but I thought there was a lot to be proud of.”
Castleford Tigers: Qareqare, Jordan, Blair, Foster, Clare, Sadler, Richardson, Hepi, Robb, Millington, Sene-Lefao, Martin, Bienek. Subs Peachey, Hall, Rusling, Magee.
Salford Red Devils: Escare, Sio, Inu, Costello, Williams, Patton, Atkin, Johnson, Ackers, Ormondroyd, Livett, Lannon, Burke. Sub Pauli, Roberts, Davies, Norman.
Referee: Tom (Leeds).