It would open the way for a "fresh face" - and Mr McDouall had some ideas about who should take over as leader, with Mr Shearer's deputy Grant Robertson, David Cunliffe, and first-term MP Andrew Little all possibles.
Mr McDouall, who stood unsuccessfully for Labour in the Whanganui electorate in 2008 and 2011, also said Jacinta Adern and Shane Jones could be "dark horses" in the leadership race.
Whanganui National party MP and government minister Chester Borrows said Mr Shearer was "a real nice guy" but lacked "political nose and political judgment".
"John Key and the Government have ratings still at election level and the Greens are eating into Labour, so they had to do something drastic to gain some traction."