Rot is a strong word and I wouldn't call it that. We won two titles after Ronaldo left and made it to the Champions league final in one of the seasons. However, I do think that our slow decline started around the time. And it wasn't only because we failed to replace Ronaldo adequately, it was also our inability to find a proper central midfielder to take over from Scholes and have a settled defense.
People can call it a title winning team all they want but our team wasn't top notch when Sir Alex retired. The central defenders Sir Alex bought (or brought up from the academy) failed to properly take over from Rio and Vidic. Jones, Smalling and Evans, though talented, were also blighted by injuries and failed to form a central defensive partnership that we had been used to till then and built our success around under Sir Alex all those years.
Evra was on a permanent decline and his back-up was one of the bigger clowns we have had at this club. Buttner I bet still cannot believe that he lined up for Manchester United at one point in his life. Rafael, as talented as he was, was still injury prone. We could hardly ever field a consistent back four because of all these talented but injury prone players at the club.
Sir Alex did try to strengthen central midfield but was mostly unsuccessful. Carrick was the last successful player he bought for the position. Anderson, Hargreaves, Gibson, Cleverley - all proved to be major disappointments. Fletcher had the misfortune of suffering from a debilitating illness.
Our wingers, Nani and Valencia, showed early promise and were brilliant in 2010-11 season (if I am not mistaken) but failed to show the consistency and quality needed. Young was a disappointment too. Certainly not someone who helped us compete with prime Barcelona. Rooney was clocking miles and despite his productivity his overall play wasn't up to the scratch in this decade.
The last title we won was solely due to Sir Alex's brilliance and sheer will. He extracted the last ounce of juice from his older, experienced players and made a brilliant purchase in Van Persie to get us the all important goals. I don't think any manager in the world, not even Jose, could have come in and won the title with the squad Sir Alex left.
Moyes was not wrong when he said that this team needed 5 or 6 quality players before it could compete for the title again. I think the club also realized that and were ready to spend money to augment the squad. Fabregas, Kroos, Bale - we were targeting the right profile of players. The only problem was that Moyes was so dull and uninspiring that no top player wanted to come and play for him. Only two players he could get were Fellaini and Mata, not game changers we needed at that time.
Van Gaal had the confidence and the charisma, but his dealing in the transfer market were atrocious. He would get a player and then discard him for not fitting his system, instead of targeting players to fit his supposed brilliant system. In all shit dealings, we did find a good player in Martial and he gave Rashford is debut. The only two positives from his reign in terms of squad improvements.
With Jose, imo, we finally started to improve our squad. Our central midfield has improved leaps and bounds with the addition of Pogba and Matic. Lukaku despite his flaws is the best striker at this club since Van Persie. Bailly, if he can stay fit, can be our central defender for a decade. Some of the buys like Mkhi and Lindelof have been disappointments but still we do look a lot better than we have in any time since Sir Alex left. That is the reason I am patient with Jose. He is not my ideal Manchester United manager but he is a manager who goes to desperate big clubs in decline and gets them winning again. I am quietly confident that he'll improve the squad further with his buys this January and in the summer.