The former Hull KR half-back, 29, had a microfracture of his left one after being injured during the warm-up in their Boxing Day friendly against Featherstone Rovers.
That meant six months out of action yet, when he was back to fitness, Ellis started suffering pain in his other knee and needed minor surgery that saw him miss another four weeks.
He finally got back on the field in Saturday’s 20-6 victory over London Broncos and, with just four regular rounds remaining, understandably hopes to get the nod again at one of his former clubs Huddersfield Giants on Sunday.
“It was a massive relief to get there out at the weekend,” Ellis told The Yorkshire Post.
“It’s been a long time coming; getting injured that early on was a bit of a low blow just before the season was due to start.
“And then to have problems with the other knee when I did get back it was gutting.
“But I’m back now. It went alright (against London). I was a bit rusty but that’s to be expected; it’d been about 12 months since my last game.
“Hopefully I can be involved again this weekend and at least finish the year strongly.
“We’re looking to get that top-five spot sorted – and then anything can happen.”
Ellis rejoined Castleford for his second spell there at the end of 2017 having helped Hull KR win promotion while on a season-long loan from Huddersfield.
But with England scrum-half Luke Gale also ruled out long-term in pre-season due to a ruptured Achilles, no one has nailed down that spot alongside Super League’s Young Player of the Year Jake Trueman.
Ellis hopes to do so now and admitted: “There’s obviously an opportunity there.
“There’s been a lot of swapping and changing between Cory (Aston), Pete (Mata’utia) and (Jordan) Rankin all filling in there.
“Truey has been doing so well – this year and last – but he needs help with a natural half like Galey or me guiding him around.
“He’s only a young kid. Hopefully I can help do that. I just want to concentrate on finishing this season strongly and building into next year.
“It’s a cliche but I do just have to take a game at a time for now.”
Huddersfield are in desperate need of a win sitting just two points clear of bottom with both Hull KR and Wakefield Trinity.
Ellis, who joined them after playing at Wembley with Castleford in 2014, said: “They are a good side.
“It’s just that it is so close down there between the bottom and mid-table.
“Wakefield are down in it now as well but you can move away quickly and we know they’ll be desperate.
“But we need another win to keep on for that top-five and the play-offs.”
It is similarly tight there; Castleford, in sixth, are locked on the same points as Salford Red Devils in fifth and Catlans Dragons in seventh.