Lomax, 22, last featured in the 52-18 win over Wakefield a month ago, and Saints have only won one of their five games since then.
Head Coach Nathan Brown told the club’s official website: “Jonny Lomax won’t be back for around six weeks. He has some fluid that is leaking into his injury (hamstring) so that is a set back.
“He was training on Saturday and it flared up, but there’s no point talking about players that aren’t going to be playing – we need to focus on the ones that are.
“He did well at full back when he was in there but Tommy Makinson is improving there and that is key for us. The blokes that come in are improving on a weekly basis and the team needs to adjust to what they bring.
“We’ve had our share of misfortune but that’s the way it is and we move on. It’s not ideal to lose players but it’s a part of the game and probably better now than in 10 to 12 weeks.
“The good thing is we’ve held up under a lot of pressure and competed hard. We’ve also created chances to win and haven’t taken them. The fight and commitment is there but we have to tighten up.”