Nelson said that WPP's almost £1bn bid would be "more innovative, more imaginative, more client-oriented than a merger of TNS and GfK".
In an interview with Research Magazine, she said: "My first impression was that the men in grey suits are winning out. This is the accountants at work. It isn't the best thing for a market research company."
Nelson, who is now executive chairman of mobile and youth specialist Q Research, said that Sorrell could turn around a business that was currently languishing and that lacked research expertise on its board.
Nelson told Research Magazine: "What Sorrell has, unlike anyone else I know, is the ability to bring companies together, and in particular ad hoc research companies.
"It's an art -- there's an awful lot of skill in putting companies together, and Sorrell has that ability.
"Sorrell in my experience has consistently spotted talent, and there is a lot of talent left in that organisation.
"Martin's wanted TNS not for 10 but for over 20 years because he knows the people, and good on him."
The former executive at the market research firm left 16 years ago and no longer has any significant holding in the firm, but her name still remains above the door and her comments are likely to sting.
Meanwhile, Sorrell is considering his options about what course of action WPP will take next. It has already made two bids, the last of which was for £996m.
Sorrell told the FT: "There's a 30% probability we walk; a 30% probability we stay; a 30% probability we might go up and a 10% chance we go hostile.
"Liz is a founder, has her name over the door, and, as a result, has the future of the business at heart."