Shaun Maloney and Scott Brown are fit again after recovering from tendinitis and an ankle operation, respectively, but will not return to full training until Monday.
New signings Thomas Rogne and Jos Hooiveld both played 45 minutes of the reserves' 2-1 victory over Blackburn on Thursday but are not yet ready after a six-week lay-off.
And while Celtic have agreed a fee with Brondby for Denmark striker Morten Rasmussen, nothing will happen on that front until next week.
All of which has combined to leave Mowbray digging deep in to his reserves to fill the bench.
Celtic beat the Saints 5-2 in Glasgow earlier in the season but the Hoops boss is expecting a far tougher game this time round.
"St Johnstone have impressed me and they have a bright young manager in Derek McInnes," said Mowbray.
"While we ended up scoring a few goals against them earlier in the season it was an entertaining football match.
"They have done well since, they have picked up points along the way and so we are expecting a very, very tough game.
"It becomes more difficult when you see our injury list - the quality footballers that will not be available to us.
"That will make it more difficult for us.
"We would still hope to go there and be competitive and get a result."
Georgios Samaras, who will start in attack alongside Marc-Antoine Fortune, is hoping to keep the pressure on SPL leaders Rangers.
"St Johnstone are a good side," said the striker.
"We must be careful.
"It is their first year in the SPL and they have some good players.
"When you play these kind of games it is always difficult and you have to be really focused from the first minute.
"But we will try and play well and win the three points."