The Welsh international who has joined Cardiff City on a season-long loan was quoted as saying he would be "glad to see the back" of Robson.
But the Baggies boss said: "I hope Jason goes on to prove he is a good footballer at Cardiff.
"I hope he prospers in his time there and I wish him good luck. Unfortunately I've got players here who are ahead of him in the queue for a first-team place."
As part of the loan deal Cardiff have first option on signing Koumas permanently for £1.5 million with a further £500,000 due in installments.