The 28-year-old was set a deadline of 19 May to decide whether to commit to a new deal at the Sixfields Stadium.
Sampson told BBC Radio Northampton: "We need to move on and concentrate on getting other players in.
"I don't know if it'll be a good decision for him. We offered him a fantastic deal and I don't think he'll find a better club at this level."
The former Swansea player scored 17 goals in 43 games for the Cobblers this season and has found the net 39 times since joining from Millwall in January 2008.
Sammo's comments make a lot of sense. Obviously don't want another Hughes/Coke/Jackman situation
Erith Cobbler
Sampson says they will be looking for a replacement as soon as possible.
"It'll be a target man, and if they can contribute the [same number of goals] Akinfenwa scored, that's good for us," he said.
"Big players have left clubs before. He isn't the first and he certainly won't be the last."
Source: BBC Sport
Source: BBC Sport