Guardiola's side have only dropped points against Burnley and United since the New Year, while marching into an explosive Champions League quarter-final with Liverpool that saw them knocked out earlier this month.
It is Abu Dhabi-owned City's third title in seven seasons and their first since former boss Manuel Pellegrini achieved the feat in 2013-14.
Despite losing in last weekend's Manchester Derby, City have wrapped up the League in joint-record time, matching United's 2000/2001 title win.
"Everyone is perfect, apart from Romero", said Mourinho in his press conference ahead of the game. "We have another chance but when one team in November, December has to hear every day they are already champions, it is so complicated to focus on what we have to do".
For a long time it has been a matter of when and not if City would win the league. Sanchez tried to strike the rebound but again Foster reacted immediately.
Guardiola might not have seen Jay Rodriguez's 73rd-minute winning goal at Old Trafford because he was due to be playing golf with his son instead of watching the match. Mourinho attempted to end the stalemate - and the tedium - by introducing Anthony Martial with half an hour remaining and his introduction undoubtedly lifted the quiet Old Trafford atmosphere.
"Bong told me that JR had said the word 'black' and then something else, but it was not clear what GB said as he was talking so fast and with such emotion".
A number of stars are expected to be axed, including Smalling, Luke Shaw, Phil Jones, Matteo Darmian and Daley Blind. I think United will win.
United midfielder Nemanja Matic had headed a corner across his own goal area and Rodriguez reacted quickly to nod home the winner.
Rodriguez, who has played for England, denied the charge and requested a personal hearing which took place at the FA's headquarters at Wembley Stadium on Wednesday.