Louis van Gaal has admitted Manchester United played "very badly" during the first-half against West Ham United but praised his players for the reaction to Cheikhou Kouyate's goal.
After a poor first 45 minutes, Kouyate put the Hammers ahead with a sublime volley.
West Ham looked set to secure three points but the introduction of Fellaini with just over 15 minutes remaining allowed United to rescue a point thanks to Daley Blind's goal in injury time.
"We played very badly in the first half and showed a lot of spirit in the second half. We gave away that goal and then we played football but you have to do it from the first minute," the boss said.
"But what spirit from my team after the goal and I thanked my players for that. [In the first half] when we had the ball we didn't play it along the floor. When we lost the ball, you have to win the first and second balls much quicker," to Sky Sports.
United ended the game with more direct football after the introduction of Fellaini.
When asked about the Belgium international impact into the game, he said: "With Marouane Fellaini you have to change your playing style.
"That's what we did and because of him we created lots of chances. I'm pleased that we have a draw here."
Questioned during the press conference about the reasons behind the first-half poor display, he added: "Especially when we had the ball, we didn't play it along the floor but also when we lost the ball, then you have to win the first and the second balls much better than we did in the first half.
"Because they were playing long balls, we knew that in advance, but all of the second balls were for West Ham and that is why we were under a lot of pressure. That is also why the players lost their confidence to play the ball along the floor when we were in possession."