Sunderland have made serious improvements in recent weeks.
The Black Cats had just three points from eight games when Allardyce took charge, but they have started to show signs of real progression.
Sunderland are now just one point away from safety, and Merson feels that Defoe deserves just as much credit as Allardyce for helping the North East side up the table.
“He’s a two chances one goal striker,” the former Arsenal midfielder said. “We talk about the Sam Allardyce factor, but all those teams at the bottom haven’t got a Defoe.
“For me, he’s a top top finisher. It’s the Defoe/Sam Allardyce combination that is the difference.”
Defoe has hit 13 goals in 24 appearances for Sunderland so far this campaign, putting him well clear at the top of their goalscoring charts.
When Allardyce took charge there were doubts over whether the veteran forward would play much of a part, but he has established himself as the Wearsiders’ main attacking threat.
Defoe will be required to keep firing Sunderland up the table, though, as they are not out of trouble yet.
Allardyce’s side are still in 19th place in the Premier League table, and they are one point adrift of safety.
Sunderland take on Crystal Palace later this evening, knowing that a victory could take them out of the drop zone.