The Scotland international today joined the Perth club on a short-term deal until the end of the season.
The 29-year-old former Hibs and Celtic striker, the third highest scorer in Clydesdale Bank Premier League history, was a free agent after leaving Chinese Super League side Shaanxi Chanba in November.
Lomas said: “We know what we’re getting and it suits both parties as it gets Derek playing again.
“He knows what the SPL is all about so we will take it from there. He will have to fight his way into our team and that won’t be easy.”
Riordan could make his Saints debut against his former club when they travel to Celtic Park tomorrow but Lomas has yet to decide whether to add Riordan to the squad.
Riordan told the club’s website he is keen to deliver goals for high-flying Saints. “It’s been frustrating not playing and I was pleased to be given the chance to train with Saints.”