Matic played a key role in Chelsea landing the Premier League title last season, but the Serbian was allowed to join United in a £40m deal ahead of the 2017-18 campaign.
Gerrard has paid tribute to the 29-year-old's qualities, and admitted that he was flabbergasted that Chelsea decided to sell the midfielder to a direct rival for the Premier League title.
"[He's] one of the toughest opponents I've played against as a six," Gerrard told BT Sport. "He's very leggy, he's good in the air, he competes, he always gets a toe in, he can run and match your runs.
"He's very aggressive with the ball and without the ball, he can burst past you with it and he can catch you if you manage to stride past him on the odd occasion.
"Couldn't believe [that Chelsea had let him go]. I think out of all the Manchester United players I've seen them sign over the years he's the one, as a Liverpool fan, I thought 'Oh no. I hope not,' and then it happened."
Matic has played every minute of United's Premier League and Champions League campaigns this season.