Morikawa, 26, was two-strokes back of the leaders in a tie for ninth after 54 holes.
After rounds of 71, 73, and 68, he was at 4-under par heading into Sunday's final round with a solid shot at contending for the win.
This week's PGA Tour tournament is a designated event, meaning $3.6m goes to the winner from a $20m purse.
Morikawa was searching for his sixth career PGA Tour win, and first since the Open Championship in 2021.
However, he was forced to withdraw from Sunday's final at Muirfield Village Golf Club with back spasms, despite being in contention.
The California-born golfer is not in the field for next week's RBC Canadian Open, but he is scheduled to play in the following week's US Open.
The third major championship of the 2023 season is also being played in Morikawa's home city, at Los Angeles Country Club.
He'll have just over a week to recover in time for the USGA's event.
Three players were tied for the lead after three rounds in Dublin, Ohio for Nicklaus' annual tournament.
Among them was World No. 3 Rory McIlroy who is searching for his 24th career PGA Tour victory.
With a win, he'd be the second player from Holywood Golf Club in Northern Ireland to win a tour event on Sunday.
Just two boys from Holywood Golf Club @tommckibbin8 is tied for the lead on -6 at the Porsche European Open.@McIlroyRory is tied for the lead on -6 at the Memorial Tournament. #PEO23 pic.twitter.com/ibLm8ZYvNK— DP World Tour (@DPWorldTour) June 4, 2023
McKibbin grew up playing at the same club that famously produced McIlroy.