He was a peripheral figure at United last season in Jose Mourinho's first year in charge, starting only 15 league games, and wasn't included in the national team's most recent squad.
The 119-time capped England international is also a former PFA Player of the Year, FWA Footballer of the Year and Premier League Player of the Season.
United will be eager to push on this season with Lukaku as their frontman, and challenge for the Premier League title.
The pair even posted a video of Pogba welcoming Lukaku to Old Trafford with the midfielder saying he would see him in training. Herrera said in May that he's not ready. "I always said I want to play for a team that's challenging for every trophy that there is".
"His main quality is his quality on the ball, his experience and what I describe as his winning mentality", Koeman told Evertonfc.com.
"Here and there he sends a bit of advice to me, and it helps me improve". But, you know, I have to work harder than I'm doing now. If he faces a battle to justify it, Koeman and Walsh's often admired judgment will be called into question if he loses it. It is a comeback with the possibility to rebound on all involved.
Ed Woodward, United's executive vice-chairman, added to the plaudits.
But Manchester United's record goalscorer did have a way to secretly continue his support for Everton.
Lukaku has already passed a medical in Los Angeles and a formal announcement is expected from United and Everton in the coming days.
Even with Champions League qualification riding on the result in the final, the United manager stuck with his No. 2.
"He goes having created some of the most magical moments in some of the most successful years in the club's history".
United have agreed to pay Everton £75m upfront plus £15m in add-ons.
Romelu Lukaku is relishing the prospect of becoming a Manchester United player, declaring he is joining "the biggest club in the world". Koeman's candour is riskier with a player of Rooney's celebrity. It's a club that's hungry to win the league again, to be dominant in the world again.
Barry has been with Everton since his move from Manchester City in 2013, making 153 appearances across all competitions.
"I'm really delighted to hopefully be part of their history".