The talented teenagers have been playing for Luton Reserves and are sought after by the Blue Square Premier side but Castle believes they would be better off by playing first-team football for City in the short term.
Castle said: "Where do you get another Poku and O’Donnell from? They are irreplaceable. Will it affect us if they go? Of course it will, but unless you are a rich club you can't stand in their way. The one thing I have is lots of contacts and they will get limitless opportunities if they stay here.
"It's down to the individual. I don't think they want to leave St Albans. I know for a fact they love it here and that's making the decision harder."
The Saints have offered midfielder Poku a deal to stay at Clarence Park but Castle is uncertain whether the 19-year-old can be persuaded against an immediate move into professional football.
City's boss said: "[Godfrey] He has to trust me and be patient. They are very interested in him and we've offered him something which is a good offer. But Luton being a professional club is turning his eye. He is doing fantastic but if he is patient there are other clubs more than interested in him.
"He has had the offer for weeks and it is down to the lad. We can't offer him full-time football and he is not working so might want to take the opportunity to play in a higher league now.
"He has had his eyebrows raised and it's very tempting for him to go for that first option. But he wants first-team football and can he get that at Luton?
"He has come an awful long way and he will play at a higher level. I rate him very highly but it's very hard to say what level he can go to. But he is showing great levels of performance.
"I think it would be very premature if Godfrey went to Luton because no disrespect to them but I know there are bigger clubs sniffing around. To me, I've been in the game a hell of a long time and I worked with Chris Bart Williams a few years back as a 16-year-old at Leyton Orient and he’s right up there with him.
"It's a dilemma he's got and I wish him well because he is one of the nicest lads I've worked with and hopefully it swings in our favour."
The Saints earned a 1-1 draw at second-placed Dover on Saturday. O'Donnell fired the visitors in front from a free-kick but the lead lasted only three minutes.
Castle added: "We had a real skeleton squad and were down to the bare bones but the lads that we had in the squad did us proud. We've played the second placed side and hopefully that gives us the belief to take on the majority of teams and give them a game and give a good account of ourselves.
"It was a great ball by JJ and we've been working on getting bodies in the box. We've been half an inch away on two chances with Gary Cohen and Adam Everitt. To come to a club with a budget of this size and with a fantastic set-up, we've got to be pleased."
The only sour note from the match was the late dismissal of assistant manager Mark Peters who was sent off for a late challenge.
"From where I saw it, he did get himself done," Castle admitted. "He got a nasty cut on his shin but Mark has got to curb things and it wasn't the best of challenges."
City have a busy programme over Christmas with a trip to Maidenhead United on Boxing Day followed by a home game against Welling United on Monday.
Castle said: "It's a very close league other than Newport. Everybody is beating each other and dropping points. It is my job to be optimistic and keep pushing us.
"I'm really pleased with the atmosphere in the dressing room. Maidstone and Welling are around us in the table and we need to go and get as many points as we can from them."