Rosenior explained his thinking after the FA Cup win over Charlton in his first game in January. "This is a team that won the Club World Cup. That was five months ago. This is a team that won the Conference League last year. They've been well coached."
The new head coach felt Enzo Maresca had left strong foundations tactically. Looking back, his early results bore it out. Chelsea won eight of those first 11 games under Rosenior, their only defeats coming against Arsenal in the Carabao Cup semi-final.
There were some adjustments to their approach during that period. Rosenior's preference for a far more aggressive man-to-man press than his predecessor was apparent in spells. But overall he favoured a light touch as he navigated their hectic schedule.
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That schedule eased after their FA Cup win over Hull in February, when the fixture list allowed an eight-day break between games.
Finally, a chance for Rosenior to properly implement his own ideas.
"I said to the players in our team meeting on Thursday morning that it was the first time we've had a two-day tactical lead into a game since I've worked with them," he said in his press conference ahead of their subsequent game against Burnley at Stamford Bridge.
"It's been one day until now. Walk out, walk through certain situations and scenarios, rest, and play," he added. "Thursday was the first opportunity for us to talk about our style and what we want to put into the game, and then actually work on it.
"Every coach would say the more time you get with the group, the more you can imprint what you want."
The problem was that the more time Rosenior got with the players on the training pitch, the worse Chelsea seemed to become.
The Burnley game ended in a bitterly disappointing 1-1 draw. Two months on, Rosenior has been sacked having only won three out of 12 games since the victory over Hull. And two of those victories came against lower-league opposition in Wrexham and Port Vale.
He leaves with Chelsea having lost five consecutive league games without scoring for the first time since 1912. He is not solely to blame, of course. Chelsea's plight is a story of mismanagement from the top down. But the pattern of performances worsening the longer they spent on the training pitch was damning of his efforts.
After the Burnley game, Chelsea suffered their third defeat to Arsenal under Rosenior following the February international break. The losses to Manchester City and Manchester United in April came after uninterrupted midweeks to prepare.
The schedule cleared but performances became more muddled.
By April, the cracks had started to appear, with Enzo Fernandez having made his come-and-get-me plea to Real Madrid during the March international break and Marc Cucurella having publicly questioned the decision to sack Maresca while away with Spain.
Rosenior's authority was also undermined by what happened on the pitch. His commitment to an aggressive, man-to-man marking system was made to look naive in an embarrassing 8-2 aggregate loss to Paris Saint-Germain in the Champions League.
Malo Gusto's comments after the five-goal first-leg defeat suggested some players agreed. "In the end, I think we need to recognise when it is important to sit back, to wait for them, to not concede too many goals," said the defender.
In between those two fixtures against Paris Saint-Germain, Newcastle were able to exploit the same flaws, when confusion over who was responsible for pressing Tino Livramento left the full-back free to drive up the pitch and set up the winning goal.
"They were playing man-for-man, so we knew we could get behind them," said goalscorer Anthony Gordon after the game.
Rosenior described it as a "new way of pressing", meanwhile. "We press in a way that is different to most teams," he added. But his side were showing precisely why employing the approach all over the pitch, rather than only in certain zones, it is not more popular. It only takes one lapse for such an aggressive press to be undone.
Rosenior did not help himself at times.
His furious reaction to an Arsenal coach encroaching into Chelsea's half during the warm-up before the Carabao Cup semi-final second leg brought unnecessary attention. The introduction of pre-match huddles in the centre circle was made to look silly too, culminating in the farcical sight of referee Paul Tierney becoming surrounded before the Newcastle game kicked off.
Rosenior's comments about "respecting the ball" brought more ridicule.
Chelsea have failed to score in six out of seven games in all competitions since that incident, the only exception being the 7-0 win over League One Port Vale in the FA Cup. Rosenior said he felt no disconnect with his players after the latest blank against Brighton but their "indefensible" performance suggested otherwise.
Rosenior can justifiably argue that the job he took on, mid-season, at Chelsea was always likely to be challenging. He inherited a squad with clear flaws. The club's recruitment has left a glaring lack of experience and leadership among the players.
But Manchester United make for an uncomfortable comparison.
Michael Carrick has not had to contend with such a hectic schedule, in fairness. But he has used his time on the training pitch to make his side better. For Rosenior, the opposite appears true.
The Premier League table since January 17, when both Rosenior and Carrick oversaw their first games, has Manchester United top, on a clear upward trajectory, while Chelsea are 10th, a side going backwards under a head coach for whom time has run out.
