The race for second or third place is still there for the Blues, who before Saturday's kick off were the only side capable of reeling in Miramar, with a game in hand.
It is understandable Robertson and his men are "disappointed with the performance and the result".
"We don't want to start making excuses about the Chatham Cup but it's not the standard we set ourselves," the centreback said, after they went into halftime 2-1 up.
English import Ryan Tinsley and teenage striker Angus Kilkolly, sitting on four yellow cards, didn't get on to the turf at all.
Enough said.
"We had a little inexperience in the team," Robertson said, after teenager Logan Bensemann came on in the last 15 minutes as a defender.
"With the squad of players we have, we should have finished better."
Well, add to the unavailables the season's best player, Welsh import Miles John, who was still trying to shrug off the effects of a concussion from the previous cup match against Island Bay United a fortnight ago, and Rovers did have an anaemic predisposition on Saturday.
Needless to say, Robertson eventually got around to what mattered most.
"It is now good to have a full focus on the cup," he said, adding it was no doubt the bigger item on the agenda of a club yearning to add more silver to its mantel piece.
"We can forget about the league this week."
On Saturday, Stu Wilson broke the deadlock in the 17th minute, taking on the goalkeeper Daniel Clarke one-on-one before going around him to rattle the gaping net.
The hosts levelled the ledger 1-1 through Josh Ebert in the 28th minute when he swept in a freekick past goalkeeper Jonty Underhill.
Seven minutes later Josh Stevenson nudged the visitors ahead 2-1, after a teammate flicked in a long throw to the front upright, where the policeman tapped the ball in.
Controversy arose in the 61st minute when right wing Benn Dawson challenged Underhill following a freekick but referee John Rowbury waved play on as the ball rolled into the net for 2-2.
"We thought it was probably a foul," Robertson lamented.
The winner came from Taban Makoii in the 82nd after Underhill mistimed a goal kick which the African striker gleefully deposited past the keeper with interest.
Was the result an upset?
Petone goalkeeping coach Barry Pickering begged to differ.
"I wouldn't say so ... we're capable of being up there," Pickering said, having beaten Miramar this season and losing only to Upper Hutt City in the lower echelons.
"Napier were looking quite tired. In the last 20 minutes they had started flagging," he said, adding that with the injection of veteran Paul Whitmarsh with 20 minutes to go in his 200th match for the club, the shape of the game had changed considerably in the hosts' favour.
The last goal, he believed, was unfortunate but accepted the Rovers had an eye on the cup.
"It'll be tough but we wish Napier all the best next weekend because we want a Central League side to win the cup," Pickering said.
Result: Petone 3 (Josh Ebert 28th, Benn Dawson 61st, Taban Makoii 82nd) Napier City Rovers 2 (Stuart Wilson 17th, Josh Stevenson 35th).
Halftime: 2-1.
