New Zealand batsman Young and Australian seamer Kelly were both in action in their respective homelands up until Monday.
Young was playing for his country - and scoring two centuries in three matches - in a One Day International series against the Netherlands.
While Kelly was helping Western Australia win the Sheffield Shield Final at the WACA in Perth by claiming a winning draw against Victoria in the country's first-class cricket showpiece occasion.
Both will miss Thursday's season opener against Gloucestershire, and that means two other players will get the chance to lay down a marker and give Sadler something to think about.
"They will both definitely be here for the second game," said the head coach.
"We have said all along it is about a squad, so this will give other people opportunities to try and grab that position that Will is going to take when he gets here.
"It is what it is, but Will is in fine form out in New Zealand.
"I have spoken to him regularly over the past couple of months, and he is in a great place and looking forward to coming and joining us.
"I am sure he will be a calm voice that we will all learn from, and his performances will I am sure speak for themselves.
"We are looking forward to getting him with us, and he is going to be a great asset for the team, as will Matt.
"It was always going to be touch and go whether he would be here for the first game, but Western Australia qualified for the final of the Sheffield Shield out there.
"That means he is going to come in a week later and means he will miss that first game.
"It is not ideal, but we fully respect that he is playing in a final out there.
"It does mean we know that when he gets off that plane he is going to be ready, he will be fresh and he will be ready to go, and by game two it will give us a bit of a lift."
The other advantage for Sadler as well as Young and Kelly is that Northants aren't involved in the second round of Championship matches.
Once the Gloucestershire game is out of the way they don't play again until they take on Yorkshire on April 21, and that will give both players the chance to get used to life, and the weather, in Northampton.
"When you are travelling half the way round the world is going to take that little bit of time just to get acclimatised," said Sadler.
"Having the second week off does give us that bit of time, and we are looking forward to getting them both in.
"They are going to be brilliant additions to our squad."