The 26-year-old Netherlands international midfielder managed only five senior appearances and just two starts for the club last season following his £6 million switch from Ajax after he suffered a serious thigh injury and then -- as he was about to return to action -- a collapsed lung.
De Jong was missing once again at Swansea at the weekend with a shin problem, but McClaren, who knew the midfielder from his own time in the Netherlands with FC Twente, is sure he has a good player on his hands when he is fit.
He said: "He can assist and he can score, and that's a big weapon to have, those two things. He's going to be a big asset for us.
"I remember when he came here, I remember thinking what a good signing that was. He's yet to light the place up, but that's purely because of injuries. He's just building up to get that chance.
"I think Siem has found that year out catching up with him, shall we say. He's going to take a while getting to the right tempo and the right level, but we can see it every day, that he's improving, getting better, and he can see that, too.''