The former Leeds Rhinos, Featherstone Rovers and London Broncos back-rower began training with Castleford last month after the old Bradford Bulls club ceased trading.
He has now recovered from a groin injury which restricted him to just 20 appearances for London last year, but suffered pectoral muscle damage playing for League One club Oxford last weekend.
Castleford coach Daryl Powell said he is “seeing how it goes with him”, but confirmed Foster could “potentially” be offered a two-year deal.
“I really like the look of him,” Powell said of the 23-year-old.
“I watched his game against Oxford and I thought he carried the ball strong and got in good areas defensively.
“He hadn’t played for eight months so it was pretty tough for him, but I thought he did well.
“I really like him, I think he’s a player who could have a fair bit to offer us.”
Powell added: “We just need to see him play a little bit more.
“I liked what I saw of him at London and he was great for Featherstone.
“I think he’s a player who has got a decent future ahead of him, he just needs to have some luck with injuries.
“That’s been tough for him over the last year or so.”
Powell is also keeping a close eye on former Hull KR forward Kevin Larroyer, who signed for Castleford in February and was immediately loaned to Bradford.
The initial month’s loan has now been completed and Larroyer will be staying with Bradford on a week-to-week basis though he will not be allowed to play in Sunday’s Challenge Cup tie at home to Featherstone.
“I sat down with Kevin on Wednesday and went through his game,” Powell said.
“He is doing some good things. He has not had a really good pre-season so we’re just trying to sharpen him up a little bit.
“Hopefully he will play a part for us. We’ve got Easter coming up and we are going to need troops and fresh bodies around that time.”
The draw for the fifth round of the Challenge Cup will be made by Leeds-born dual-code winger Jason Robinson, along with presenter Rachel Burden, on BBC 5 Live’s Breakfast Show next Tuesday.