"But luck just wasn't on our side. We dominated in the second-half but just needed to be more accurate with our chances."
The defeat leaves United in fifth place on 25 points after 16 games.
Despite the loss Walker said there will be no problem getting his team motivated for their crucial away derby clash against top of the table Ngongotaha AFC next weekend.
"Everybody will step up their game for the occasion," Walker said.
"It's going to be a very tough game and it's always a difficult place to travel to.
"The key will be to counter their dangerous attack and put in a similar strong defensive display."
Meanwhile Ngongotaha remain unbeaten in the Premiership this season after a 6-1 away demolition over league strugglers Tauranga Boys' College 1st XI.
In form striker Mitchell Miller scored an impressive five goals, taking his tally to 33 for the season and substitute Hayden Booker also made it onto the score sheet for the Villagers.
Ngongotaha coach Shane Davis was happy with the result but admitted he had a "stern half-time talk" with the side, who went in 1-0 up at the break.
"I felt we were just going through the motions," he said.
"But then the boys responded after a firm word and started to shift the ball quicker, creating lots more chances.
"Miller once again was awesome and took the opportunities."
Ngongotaha have a nine-point cushion at the top after Matamata Swifts Prem suffered a shock 2-1 away defeat to Katikati FC Ambria Premiership.
But they may have to play the rest of the season without star keeper James Towers who suffered a suspected torn leg muscle injury on Saturday.
"It's a big loss. James has been in excellent form but we will have to deal with this and not let it affect our confidence," said Davis.
Results:
WaiBOP Premiership:
Rotorua United 0 Otumoetai FC First Credit Union 1
Tauranga Boys' College 1st XI 1 Ngongotaha AFC 6
SGS Papamoa Premiership 6 Tauranga City Premiership 1
Melville United Prem 0 Cambridge FC Prem 6
Katikati FC Ambria Premiership 2 Matamata Swifts Prem 1
Old Blues Prem 2 Waikato Unicol AFC Prem 0
WaiBOP Championship:
Te Awamutu Edmond Judd Champ 2 Ngongotaha Champ 0
Old Blues Champ 3 Waikato Unicol AFC Reserves 3
Whakatane Town Champ 0 Cambridge FC Champ 4
Tokoroa AFC 0 Champ Matamata Swifts Reserve 0
West Hamilton Champ 3 Taupo AFC Champ 2
Hamilton Wanderers Champ v Otumoetai FC Mens (postponed)