But the head coach does accept the timing of the decision, coming less than a fortnight before the start of the new season, was not ideal, nor is the fact Rossington has since decided his future lies elsewhere.
It was announced on Monday that the 28-year-old wicket-keeper batsman has joined LV= County Championship Division One rivals Essex on loan for the whole summer, effectively bringing to an end his eight-year stint at Wantage Road.
With Thursday's season opener against Gloucestershire at the County Ground looming on the horizon, it is hardly the ideal preparation, but Sadler insists he and his squad have to now 'draw a line and move on'.
Speaking to BBC Radio Northampton, Sadler was asked about the decision to change captains and the timing of the announcement, and he said: “This is not just a flash in the pan, wake up one morning and what decision are we going to make today.
“This has been building up over a period of time.”
Sadler was keen for Rossington to stay on purely as a player, but that was something the former Middlesex man did not want to do.
“We tried to work together as closely as we could with the direction we are going, but it kind of came to a head and this is where we are at,” continued Sadler.
“Sometimes you have to make a call, and what I would say is that I fully accept the timing question.
“But the flip side to that is that the cricket is here now, we have to draw a line and move on. It is what it is, and we crack on as best we can.
“It is a shame it has worked out this way, but I still stand by the decision.”
Northants have two more days of preparation before they get the campaign underway on Thursday.
Rossington will be replaced in the team by Hampshire loan signing Lewis McManus, who made his debut for the club in last week's friendly against Cardiff UCCE.
Sadler will also be without the services of overseas signings Will Young and Matt Kelly for the season opener.
Young played for New Zealand in their third and final One Day International against the Netherlands on Monday, scoring a second century in three innings as the Kiwis won the series 3-0, while Kelly was part of the Western Australia team that won the Sheffield Shield Final against Victoria.
That match also ended on Monday.
Fast bowler Brandon Glover is also unavailable as he was in the Netherlands squad in New Zealand.