If we stand still like we did after the previous two title wins expect a similar outcome. We need to strenghten, mahrez on his own wont be enough. All our rivals are strenghtening big time so expect a stronger challenge this season. History will tell you all you need to know about how difficult it is to retain the title.
I think people exaggerate standing still being the reason for us not retaining the title in 2013 and 2015. Lets be honest, there were serious off pitch issues and if Mancini hadn't started fighting with everyone, or if Yaya's brother hadn't died and Fernandinho hadn't been blamed for Brazil's worst ever defeat we'd probably have defended those titles.
It wasn't because we didn't have the squads to retain the title, it was because the squads underdelivered under managers who were in crises.
If you look back at previous title retaining sides and what they changed in the summer:
2008/9 - United get Berbatov.
2007/8 - United get Nani & Anderson who were awful.
2005/6 - Chelsea get Essien and SWP.
2000/1 - United get no one.
1999/0 - United get Barthez & Silvestre.
Teams who defend titles don't go out and buy a load of new players, 1 big signing, maybe 2 back up signings is all that you need to keep things fresh.
Mahrez will be enough. Maybe we'll get one more, maybe not, but we don't need upheaval, we just need to keep plodding on while everyone else is trying to reinvent themselves to cope with our previous season.