The 31-year-old Australian rejected an extended contract at the Riverside Stadium after manager Gareth Southgate had repeatedly pleaded with him to stay.
Boro Face A New Problem After Viduka Exit
Lamb: "How much we miss him and how well he does up the road, only time will tell.
"But we are moving forward. It is not about losing Mark Viduka, it is about Middlesbrough Football Club and how we replace Mark Viduka.
"The fans can be certain that Gareth and the rest of his staff are doing all they can to bring in replacements.
"We are doing all we can. The transfer market is manic at the moment.
"There is a lot of activity, but not much activity, if you understand what I am saying. All sorts of clubs are being linked with all sorts of players.
"It will settle down eventually. We have our targets, our positions to fill, and we are working quietly behind the scenes to do all we can to bring those players into the football club.
"I am sure there will be a number of new faces at this football club before the start of the season, and there will be some old ones who remain with us.
"It will be an interesting mix for next year, but one I am sure Gareth, come the first match of the season, will be absolutely delighted with."
Southgate revealed last month that if he had to sell players to boost his