Udoh, who netted 15 times in all competitions for Salop last season tore his anterior cruciate ligament in the 1-1 draw at Bristol Rovers last month and is facing the rest of the season on the sidelines.
He underwent surgery in the injury this week and manager Steve Cotterill has revealed it was a success.
He also explained that Udoh had no tear to his meniscus, which the Salop boss described as a ‘real bonus’ as Udoh’s road back to fitness now begins.
He said: “Dan’s surgery has gone great. I had a text last night from Dan Greene (head physio) who had a text from the surgeon saying everything had been done and he has a rock solid knee.
“It has been really frustrating as we’ve had to wait for Dan to have no swelling, and that has been frustrating.
“But I said to Dan yesterday, this is the start of his rehab now - and by the sound of it all the operation has been really, really good.
“There are no meniscus tears which is real,real bonus, and we are looking forward to seeing him.”
Fellow striker O’Brien has endured a frustrating start to life at Salop - and underwent surgery on his hip in mid August.
Cotterill gave an update on his fitness, as well as Coventry loanee Da Costa, and insisted he wants to make sure both players are right before they return, much like Elliott Bennett, who returned from an ankle injury in the midweek cup defeat to Port Vale.
He added: “Aiden has been feeling better, he is doing very, very well. With all injuries you lose muscle over night and with surgery you lose them within hours, and then it takes weeks to build back up.
“So we are just building him up so he is strong enough. He is certainly fitter, learner, lighter, his body fats are down.
“These next few days are big for Aiden leading into a bigger week next week, and we want to make sure we get him right, the same as Elliott.
“Julian Da Costa the same he is coming along. We have missed him. Before the injury came along he gave us fantastic thrust when we played 3-5-2 and we have missed that a bit."
Cotterill was also full of praise for Bennett, who made his first start in mid week since he underwent surgery on his ankle.
Bennett completed 80 minutes of the game before a scheduling visit to his surgeon - but missed training on Thursday due to sickness.
The Salop boss added: "He hasn't come in today as he felt sick this morning.
"He was fine after the game and we knew he would obviously stiffen up. He went back to see the specialist in London yesterday, not because of the game, but he was scheduled in to see him, so he has had a busy 24 to 48 hours.
"He woke up a bit sicky he might have picked up a big, we don't know, but he will be okay, for what at the moment we don't know.
"Elliott is a great lad who I have known years and years before I came here, I know what he offers.
"The other night he was excellent in everything he did, for his first game back he deserves a round of applause for that.
"I'm not putting any pressure on him. Hopefully it is only a sickness 24 hour thing."