He told BBC Sport back in March that it was "hard to take" when he was passed over in favor of Emmanuel Adebayor or Rafael van der Vaart by then Spurs manager Harry Redknapp, and when he would be left off the roster for international matches.
To an extent, Defoe is right to be upset. According to The Telegraph, he has the best goal-scoring ratio of any striker at Tottenham and scored an amazing 11 goals despite starting just 11 matches last season.
His frustrations have now reached a boiling point, and several managers are doing their best to take advantage of the player's unhappiness at his current club.
However, Lambert added that the 29-year-old is valuable enough to snatch up if the situation presented itself:
I think a club this size will always be linked with players of that ilk. I don't think you ever stop looking for players. You have always got to try and improve. Whether they come in or not we will have to wait and see, but any manager will tell you that you are always going to look for players.
Meanwhile, Defoe came out as recently as today to once again voice his frustrations that he isn't given more opportunities with Tottenham (via The Telegraph):
Are you worried Defoe will leave Tottenham for Aston Villa?
I’m not one of those players that will sulk. I just keep it going because if you sulk and then do get a chance, because your head has gone, you can’t really perform. I’ve always waited for my chance and when I get one, make sure I grab it with both hands. I do feel sharp.
In my opinion, it's Spurs and manager Andre Villas-Boas who needs to grab Jermain Defoe with both hands, not the other way around.
Defoe is the only senior striker on the books at Tottenham, and Lambert is a crafty manager when it comes to transfer windows. The former Norwich boss has signed the likes of Brett Holman, Karim El Ahmadi, Brad Guzan, Matthew Lowton and Ron Vlaar this summer as he tries to rebuild Aston Villa in the wake of a disappointing 16th place finish last season.
With most players, it's all about the money, but I don't think that's the case with Defoe—all he wants is an increase in playing time. I don't think that's too unfair of a request.