But the County head coach believes the Steelbacks skipper still has 'big ambitions' to fulfil, including bringing success to Wantage Road.
Willey bowed out of international duty in style on Saturday, claiming the man of the match award as England completed their dismal World Cup campaign with a win over Pakistan in their final group game.
The 33-year-old was one of the most impressive performers for England in India, claiming 11 wickets at 23.55 apiece in his six matches, finishing with three for 56 against Pakistan.
Those three wickets saw him go past the 100-wicket barrier in ODIs, becoming only the 15th England player to reach that mark, doing it in 73 matches.
Willey also played in 43 T20Is, bagging 51 wickets in that format, so can take great pride in an excellent England career that stretched out over eight years.
Now he will concentrate on life at Northants, having rejoined the club last winter on a four-year contract, and also some franchise cricket around the world.
Sadler is certainly looking forward to seeing more of the Steelbacks skipper around the County Ground from now on, and believes he can hold his head very high after what he has done for his country.
"David was always a bit of an unsung hero with England," said the head coach.
"He always did well, he always contributed, he was always a massive team player and as we all know he puts his heart and soul into everything he does.
"He shows full commitment and as a club we are all very proud of him.
"Retiring wasn't a decision that he came to lightly, and he had been mulling it over for a while, but I think him and his family they just decided enough was enough.
"It gives him a bit more certainty about what is going on, he has always been the last man in and first man out, and has always been a little uncertain of his place in that England side.
"There are some fantastic players in that squad and it is just the way it has been for him, but I think he is ready to say 'that chapter’s closed, and I am now going to focus elsewhere'."
And on what lies in store for Willey over the next few months and into next summer, Sadler added: "I know he is looking forward to being around with us at Northants over the winter.
"He has other bits going on and will be playing some cricket, but he is looking forward to being here.
"He is also going to be around for the full Blast campaign which is crucial for us, particularly as captain and leader, and as a senior player.
"But he has been brilliant for England, and he should be very proud of what he has achieved at international level and where his career has gone, but I know he is absolutely not finished yet.
"Dave has some big ambitions moving forward, and he is fully committed to achieving success with us at Northants, and wherever else he is going to play around the world."
Willey’s role at Northants next summer will definitely see him lead the Steelbacks in the Vitality Blast, as he did last season.
He made no other competitive appearances for the club in 2023, but Sadler is hopeful he will play in other tournaments in 2024, and he may even turn out in the LV= Insurance County Championship.
The head coach insists that Willey is very keen to play red ball cricket for his home town team, and that has always been the plan since he rejoined the club from Yorkshire last winter.
There has been plenty of chat among supporters that Willey has only signed a white-ball contract at Northants, but that is not the case.
And Sadler has confirmed that if he is fit and available, then Willey is available to play for the County in all competitions.
But he is also mindful of the fact the player will be in demand for franchise cricket over the globe – and that the club will back any other player that gets the call for the Hundred, IPL or other competition.
"He is under full contract with us, and always has been, it just didn't work out that he could play any red ball cricket for us last season,” said the Northants head coach.
"But moving forward he is very hopeful and very keen to feature at some point, so we will have to just monitor that as we go.
"We support what he does and it is about getting that balance right.
"We want to make sure that we get some cricket out of him, but respect that he has other stuff going on as well, that is what happens with international cricketers.
"I would love Dave to be around here all day, every day, as simple as that.
"But we know the global game is different now to what it once was, and if any of our players get picked up for tournaments at whatever time of year, then we encourage that.
"We will support that because it is great for the club and great for the players that these lads are going on and representing at higher levels.
"We had Saif Zaib playing in The Hundred this year which was fantastic for him and a great experience, and we want to get more of our lads doing that.
"It has to be seen as success if we bring lads through and they go on to get higher honours, just like Dave has. It’s part of our job, part of the role of the club."