The England defender has left Wasserman Media Group and will now be represented by OmniSports.
Smalling received criticism from his manager in April about his commitment, following an innocuous injury he received whilst training with England.
The Portuguese said of Smalling and teammate Phil Jones: "It's not just about them. It's about the philosophy and mentality around them. Cautious. Cautious. Cautious. Just a cautious approach.
"It's a profile, it's the philosophy of work. Just that."
"I think the fact that we are fighting for important things still is a good motivation to keep people together, to make people make an extra effort."
Smalling moved to Old Trafford from Fulham for £12m in 2010 and has since gone on to make 243 appearances for the club.