The Blues had a second offer believed to be in the region of £30m rejected earlier this week, with United adamant that Rooney is not for sale.
Robson believes that Chelsea's bid is derisory and gives United no chance of replacing a proven goalscorer at a similar price.
He told talkSPORT: "The thing that makes me laugh is I've been told it [the latest bid] is £25m to £30m. For Wayne Rooney? A 27-year-old who is probably in the top six strikers in the world?
"What are United going to have to pay to replace him? I'm amazed at that. I think it is an insult to Wayne Rooney. I don't know what's in the head of Wayne and his agent, why he wants to leave. I know he had loggerheads with Sir Alex Ferguson but David Moyes is the manager now. To me it is so important for him to concentrate on his football this season.
"I don't know what he sees, the grass being greener on the other side. The fans love him, the players love him so I don't see what the unsettlement is. Everyone would like to see Wayne Rooney settle down and be part of the team to try to retain the title."
Rooney is yet to feature in pre-season for the Red Devils, with a shoulder problem the latest reason given for his omission from United's friendly against AIK Fotboll on Tuesday.