Saints boss Steve Lomas is banished to the stand, while defenders Dave Mackay, Frazer Wright and Steven Anderson are suspended along with midfielder Chris Millar.
"They do have defensive problems, because they have four players suspended for the game," McGlynn said.
"From the midfield forward they've still got a lot of attacking options, so we have to make sure that we don't think just because St Johnstone have got a few suspensions it's just a matter of turning up.
"That can be costly. We've got to make sure we have the right attitude.
"We won against Aberdeen last week and we look to go up to Perth and play as well if not better and be as committed, if not more, and win that football match."
Against Aberdeen, McGlynn changed tact and started with John Sutton and Gordon Smith up front in a 4-4-2 formation. Hearts could continue with that line-up this weekend, although at least one change in personnel will be necessary with midfielder Mehdi Taouil out with a groin problem. McGlynn added:
"It was good to get back with a victory against a decent Aberdeen side, as was shown in midweek with their good cup victory against Motherwell.
"We want to try to build on the victory we've just had. We're looking for consistency."
Kevin McHattie has slotted in seamlessly at left-back following Danny Grainger's cruciate knee ligament injury. The 19-year-old was on loan at Alloa last season and is enjoying playing alongside Hearts captain Marius Zaliukas and Andy Webster, another experienced defender. McHattie said:
"It's good for me as a young lad to come through the youth ranks. This is good experience for me.
"I've just got to concentrate on myself and try to do my best."
McHattie, who moved to Hearts aged 16 from Dunfermline, has played against Celtic, Hibernian and the Dons in recent weeks.
"It's hard," he said.
"It's hard for me as a young kid coming through. I've got to concentrate and I've got Marius and Webby there to keep me right."
McHattie appears set to occupy the left-back berth for the rest of the campaign and is optimistic, if the current squad stays together of a successful season. He said:
"Top four, I reckon, with the squad we've got."