Rovers had set a target of retaining the league title, winning the Chatham Cup (national knockout) and retaining the O'Brien Challenge Shield but Miramar and Stop Out had them shuffling back to the drawing board with two wins each.
"I wasn't able to put the same team out week in, week out," he said, stressing the loss of goalkeeper Ruben Parker Hanks (8 weeks), striker Stephen Hoyle and playmaker Saul Halpin, in particular, had taken its toll.
On Saturday, striker Tom Biss scored in the 19th minute before Hoyle made it 2-0 in the 71st.
Robertson, whose coaching role entails implementing a development structure, is in negotiations with the club board to renew his contract for another season.
"After sorting out my own future I'll look at the players' futures," he said of Rovers, who also have four imports.