Callum McGregor knows there is no margin for error in Celtic's William Hill Scottish Cup last-16 clash against St Johnstone at Parkhead on Sunday.

Brendan Rodgers'  side beat the Saints 2-0 both home and away in the Ladbrokes Premiership before their 2-0 home win over Hibernian in midweek left them with a perfect  six wins out of six since the turn of the year to keep them six points clear of rivals Rangers at the top of the table

However, the 25-year-old midfielder realises any mistakes against the Perth men in the third meeting between the two clubs in less than a fortnight, could have fatal consequences with regards the Hoops' bid for an unprecedented domestic treble-treble.

The Scotland international said: "It is knockout football.

"You have to turn up and play well on the day to get through. You want to win them all but in this one you can't afford any mistakes.

"With it being the cup we are focused on not making mistakes. That is keeping the focus sharp.

"We know what they are all about, and they will have a feel of what we are all about.

"It is just a case of doing what we do, bring our A-game and hopefully get through.

"When they played at Celtic Park they sat low and when we played them away they came after us a bit more.

"So it is what they are going to do next and that gives us something to think about.

"It is probably bit of cat and mouse in terms of the tactics but we have to just look after our side, prepare properly and hopefully get the win.

"You have to turn up with the right attitude and work hard and produce on the day.

"We have been good at that over the last few years, we have really turned up when it  has mattered.

"It is no fluke, it is down to preparation and being willing to work harder than your opponent."