McGahan is keen to maintain a high level of performance, after last week's hard-fought 24-13 win over the Cardiff Blues at Thomond Park Stadium.
"The difference between the performance against the Blues and the previous one against Glasgow was attitude really," he said.
"We did not turn up to play (against Glasgow) and we were beaten physically around the park. The set piece was obviously a problem. Against Cardiff, we rectified that to a degree.
"We really have to be on our game next weekend. The Scarlets have started (the league) well."
MUNSTER Squad (v Scarlets): Backs (11) - Danny Barnes, Felix Jones, Scott Deasy, Denis Hurley, Doug Howlett, Barry Murphy, Tom Gleeson, Lifeimi Mafi, Jeremy Manning, Toby Morland, Peter Stringer.
Forwards (14) - Marcus Horan, Tony Buckley, Julien Brugnaut, John Hayes, Denis Fogarty, Damien Varley, Mick O'Driscoll, Donnacha Ryan, Alan Quinlan, Billy Holland, Tommy O'Donnell, Niall Ronan, Denis Leamy, James Coughlan.