The Italian's departure from Ducati was announced late last week, with confirmation that he would be rejoining Yamaha - and once again partnering Jorge Lorenzo – following shortly after.
On his official twitter account, Rossi posted: "It's been a real shame failing to be competitive with Ducati, it would have been a nice satisfaction for me and all the guys who have worked with me.
"They will keep on trying until the end. Plus, it would have been fun for all our fans. Shame.
"Anyway it's still eight GPs to the end, we'll work at the maximum to be able to do some good races."
Team boss Vito Guareschi said that with Rossi's decision made, Ducati would be able to focus on its on-track efforts again.
"The past month has been very intense for everyone, but now things are clearer," he said. "We can therefore concentrate on our work on the track as we continue to improve our bike so that Vale and Nicky [Hayden] can finish the season on an upward trend.
The Italian squad is yet to announce Rossi's replacement, although Tech 3 Yamaha's Andrea Dovizioso has been strongly linked to the sole remaining factory seat. Hayden is already confirmed for 2013.