The lanky midfielder was written off by a number of Manchester United fans following his £27.5m arrival in 2013 but he is enjoying a resurgence under Jose Mourinho.
Fellaini has four goals and one assist to his name this term, including a brace against Crystal Palace yesterday.
Mourinho has repeatedly backed Fellaini since taking charge at Old Trafford.
And Liverpool legend Gerrard believes the Belgium international provides Mourinho with a unique outlet.
“He’s horrible (to play against),” Gerrard said on BT Sport.
“He’s a handful, even when he was at Everton, he was a little bit further forward.
“He used to back in, get flick-ons, hold the ball up.
“The key here is that the manager loves him.
“He’s always going on record praising him.
“I think he’s been quite clever keeping him as a plan B because sometimes you can try all the class and all the nice bits, but sometimes you need to go ugly, you need to go direct, you need to have a plan B.
“He’s certainly United’s plan B and he’s very effective.”