Newcastle finished third in the English premier league last season, during which they splashed out 9 million pounds on defender Jonathan Woodgate, but the only arrival since was midfielder Lee Bowyer on a free transfer.
"If it was humanly possible, or financially possible, we could do with two more players and the chairman (Freddy Shepherd) is aware of that," Robson told reporters on Friday.
"In an ideal world, we would sign another two players when the transfer window opens. We have to do what we feel we have to do, and do it when it's necessary.
"In the summer, for us to go one better, we perhaps needed two more players in addition to Lee Bowyer, but we didn't move.
"That became a bit more scary because of the start we had, when we lost games and did not qualify for the Champions League, but we've recovered fairly favourably since then."
Robson, whose fifth-placed side visit Charlton Athletic in the premier league on Saturday, has been able to spend more than 60 million pounds on new players since taking over at St James' Park in 1999.
"At the end of last season, I don't think we wanted very much because the chairman had brought in Titus Bramble, Hugo Viana and Jonathan Woodgate at a considerable cost. He'd spent quite a bit, really, and we had a very strong squad," he added.
"But we're okay without the two players I have in mind, as Tottenham will testify. Of course, the pressure told when we lost a few matches we didn't expect to lose and found ourselves out of the Champions League.
"We've been fortunate to inch our way back up the table and we're beginning to show a little bit of what we had last year."
