Van Persie struck in the final minute to seal a 2-0 win for West Brom before lifting his shirt to reveal the message: `rip tchuna we will never forget you` and it was confirmed the message related to a close friend of the Dutchman, who died on Friday.
"He is a fantastic player. He came on and changed the game for us," said Ferguson of Van Persie. "Every time the ball came up to him he got hold of it and turned on them. They were having to go back to their box all the time and he has done well."
The United boss added: "They were a very resilient, stubborn team. They worked really hard on their defending and closed all their ranks. You have to deal with that but the slowness of the pitch killed our speed a bit."
Not that West Brom boss Steve Clarke was surprised at such heroics.
"Ben Foster made some good saves," said Clarke. "That's why we spent our whole transfer budget on Ben in the summer."
After a difficult first half, the visitors came to life after the break, triggering United into a tactical change, with Chris Smalling dropping into central defence alongside outstanding skipper Nemanja Vidic and Chris Smalling following Van Persie's introduction.
"It is not demoralising," said Clarke of Van Persie's introduction.
"The fact they had to bring some of their big guns off the bench shows how well we were doing. In the first we were too timid. We spoke at half-time about being too respectful and in the end, the scoreline doesn't reflect the way we played."