The Red Devils have been suffering all season from the failure to strengthen the squad Sir Alex Ferguson left behind.
Cristiano Ronaldo headed a lengthy list of top-name players the club were strongly linked with, and the pursuit of Cesc Fabregas was another obvious attempt to bring in fresh blood.
In the end, only Marouane Fellaini was captured - at a cost of £27.5million - and even that had to be forced through on deadline day after the Belgian agreed to pass up some bonuses remaining on his Everton contract.
The result has been a series of disappointing performances, with Jagielka the latest to profit after leading Everton to their first Old Trafford victory in 21 years.
"I have a lot of time and respect for David Moyes," said Jagielka.
"I do feel sorry for him. The fixture list wasn't that kind initially and the people who were making those decisions about transfers in the summer didn't help him out one bit.
"He has some good people around him and some great players in his squad.
"He just needs to add a few more. Once he gets that sorted and he gets more of his team rather than a team he has been given, that is the acid test but I think he will come through it, 100 per cent."
Yet Jagielka is acutely aware the expectations at United far exceed those at Everton, where Moyes made a habit of starting slowly and finishing fast.
Steering the Merseyside outfit regularly into the top eight - and above Liverpool for the past two seasons - can be achieved by dropping points early.
For United, the standards are higher.
"The pressure was always going to be high," said Jagielka.
"I am sure he knew that once he took the job. He wasn't going to get a quiet life at Manchester United.
"At Everton he could grow into the season.
"You can't grow into the season at Manchester United. You need to come out firing on all cylinders. You can't give the others too much of a lead.
"I am more than sure he will get Manchester United where they need to be. He just needs that little bit more time. But sometimes, time seems a hard thing to get at that top level."
Pressure at all levels of the game has never been more intense, nor reaction more widespread or extreme.
Given what he achieved in 11 years at Goodison Park, it was sad the vast majority of Everton fans opted to abuse Moyes, rather than acknowledge him.
"It is how fickle football is," said Jagielka.
"When you leave a big club like Everton, with such passionate fans, I am sure David Moyes expected the reception."
Though Jagielka spent six years working with Moyes, he is now working for another master.
Yet, without meaning to, the England defender underlined the difference between Everton managers past and present.
Moyes has been criticised - unfairly at times - for negativity and excessive caution, which some are starting to raise more often.
Roberto Martinez showed at Wigan he was not given to cautious thinking, even in the nerve-shredding heat of a relegation battle.
It should have come as no surprise therefore that the visitors were pushing forward at the end, just as Martinez demanded.
"The manager is ultra positive, whether that is the people he picks, the formations we play or the belief he has," said Jagielka.
"When someone has such self-confidence, it rubs off massively on his players.
"David Moyes left Everton in great shape for the new manager to come in and use his philosophies to take us to another dimension.
"He has been clever enough with his signings and the squad has different options now."
On current form, Everton's next task is even harder; Arsenal away, another fixture they have not won for some considerable time, in this instance January 1996.
"One down, one to go," said Jagielka.
"We are confident at the moment and obviously it is easier to tell someone to do something when he is happy and scoring goals or keeping clean sheets.
"The test for us - and hopefully we don't have one - will be how we deal with a bad period."