The French attacking midfielder joined Spurs from Newcastle United on a five-year deal for a fee of £30million on deadline day.
Regarding the last day transfer saga involving Tottenham and Everton, former Arsenal star Merson feels Sissoko was disrespectful to Toffees boss Ronald Koeman for failing to answer his phone after it had become apparent that he was set to move to North London rather than Goodison Park.
"I watched him at Newcastle last year, he stayed out on the wing and didn't do anything," Merson said on Sky Sports News HQ.
"When you're that good you've got to be able to do something out of the ordinary to get your team out of it as that's what top players do. I wouldn't have been putting my £30million on him.
"He was playing in front of 50,000 Geordies, if you can't get up for that, when are you ever going to get up for playing a football match.
"It's unreal he can't even answer Ronald Koeman's call. What's the game coming to? This man is a legend for what he's done in football and he can't even pick the phone up and say 'I don't really want to sign for you, sorry!'"