The Red Devils wideman faces months of rehabilitation after fracturing and dislocating his ankle during the UEFA Champions League opener against Rangers earlier this month.
Smith, a former United player, suffered a similar injury in 2006 and struggled to return to his former powers, eventually moving on to the Magpies.
The former Leeds frontman has warned Valencia he must be prepared to accept he may never make a full recovery.
Smith said in the Daily Mirror: "I've not been the same player since my injury. You've seen Eduardo have the same injury and he struggled so much and the thing is you've got to make the best of it.
"It would have been so easy to pack it in or not fight hard enough to get back and so easy to let your standards drop.
"It takes hours of work. Hopefully Antonio Valencia, who I saw have a bad injury, will be able to get back to the level that he was at before - but I know it will be very difficult. But I'd like to wish him all the best doing that."
Smith admitted the injury forced him to change his position from striker to combative midfielder, a role he now relishes.
The 29-year-old added: "I've changed my position and I'm still trying to learn that and its good for me, it's interesting.
"I played it only a few times but as I say it is good - you're always involved.
"It was like a new start for me really. You have to prove yourself game in, game out. That's what football's all about. Everyone loves playing and as long as I'm able to play at any level I'll still do so."