Hammers boss David Moyes is looking for midfield reinforcements and the 23-year-old is considered to be a cheaper option than their other targets, such as Steven Nzonzi and William Carvalho.
"There will, of course, be a lot of speculation around John regarding other clubs being interested in him," Dempster told the Daily Mail.
"There was an offer in the August transfer window and John has gone from strength to strength in the subsequent period.
"It's inevitable that when you have good people either working for you, or playing at a club, that there is going to be interest.
"It is always about weighing up our league position and what is best for us in the short and medium-term.
"We don't know what the next month will bring, we don't have a crystal ball we might get tested.
"If we are tested then we will carefully consider it."