Liverpool captain Gerrard this week confirmed that he will join MLS champions LA Galaxy on an 18-month contract in July.
The 34-year-old former England skipper has spent 26 years with the Premier League club, but opted to move Stateside after seeing his playing time reduced under Brendan Rodgers.
Former United States international Harkes spent six years in England before returning to his homeland in 1996 to play for DC United in the inaugural MLS season.
Harkes, who hung up his boots in 2003, has been surprised by the rate at which the MLS has grown and revealed that Gerrard's arrival has caused a real stir in the US.
He told Perform: "I left West Ham to come back here in 1996 and if you asked me to fast forward the league into its 20th year and I'd be talking that at this point we would have 20 teams in place, that David Beckham had already come and gone and exposed the game all over the world, that Thierry Henry played here, I would never have believed it.
"It has really surpassed the expectations of myself and it's tremendous. It's in such a great state, the league. From what Thierry Henry is able to bring to the game, David Beckham and certainly now Robbie Keane - he's been tremendous and for me the best striker in the league.
"Now you look at Steven Gerrard, he's easily one of the best players, if not the best, to come and play here.
"I don't see it slowing down, the progress has been tremendous. You see the quality of players coming in, young and old, it's going to continue to grow and people are very excited right now about Steven Gerrard."
Harkes, formerly the assistant to current Galaxy coach Bruce Arena at New York Red Bulls, considers luring Gerrard to the MLS to be a major coup for the league.
He added: "It's impressive, it's massive news here and very positive. To get a player like Steven Gerrard's status, often people say that Major League Soccer shouldn't be like the old NSL [National League Soccer] days.
"You don't want to get players that are coming over at retirement age, but Steven Gerrard has a lot of football left in him and he's a quality player.
"His discipline, on and off the field, is fantastic and his high skill level, so he's going to raise not just the awareness and exposure of the game in this country and abroad, he's also going to raise the level of play every day.
"Getting a player with legendary status from Liverpool, Steven Gerrard, I think is incredible. Not just for the club itself, but also for the whole league."
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