New Zealand batsman Young and Australian seam bowler Kelly joined in with their new team-mates for full training for the first time on Tuesday, and are both fit and available for selection for the County Ground clash (start 11am).
Kelly jetted into the UK last week, while Young arrived at the weekend, and head coach Sadler is delighted the pair are now integrated into the squad and ready to get into action - even if it does mean he now has something of a selection headache!
"It is great to have them here, they bring that experience, they bring that class and they are both really good, humble guys as well, which makes a difference as they will fit into our unit really well," said Sadler.
"We do our homework and it is really important to get the right characters involved as they are going to be with us for quite a while, and when you are living in each other's pockets day in, day out, it can be quite intense."
The County are back in action on Thursday having had a week off following their opening, and hard-fought, draw with Gloucestershire at Wantage Road.
The arrivals of Young and Kelly means that two players who took part in the opener will have to make way against Yorkshire.
And Sadler admitted: "Everybody used to say to me, that when you have a selection headache it is a good thing, and that is absolutely the case right now."
As well as having a healthy squad training at Wantage Road this week, the second team currently playing against Kent in Canterbury includes the likes of Luke Procter, Ben Curran, Josh Cobb, Nathan Buck, Simon Kerrigan, Charlie Thurston and James Sales.
And Sadler is hoping the week off his players have just had will be of benefit to the team.
"We have got a fully fit, fresh squad, who are hungry to get back out there again," said the head coach.
"You have the build up to the first game and then have no game in round two, but it is one of those things.
"But it does mean we have a fully fit and fresh squad and are raring to go, so that has to be a good thing."