Mickelson 's hopes of weekend action at the Country Club came to a screeching halt after he comfortably missed the halfway cut at the 122nd edition of the US Open.
Before making his way to Brookline, Mickelson's only competitive appearance since February was the inaugural LIV Golf Invitational Series event which saw him finish outside the top 30.
The American, who has received $200million for signing up to the Saudi-backed breakaway tour, continued his struggling form in Boston as he followed up Thursday's 78 with a second round of 73 to finish 11 over par.
"I enjoyed getting back out there," Mickelson told a group of US outlets.
"The US Open is the ultimate test and you don't really know where your game is until you get tested. I thought I was a little bit closer than I was, but I really struggled putting. I thought I was more prepared than I was.''
Mickelson took a four-month break from the sport after making explosive comments about LIV Golf and the PGA Tour.
His next competitive appearance is to take place in the second event of the Saudi-funded tour, which will take place at Pumpkin Ridge in Oregon at the end of June. The 52-year-old will then head to St Andrews for the 150th edition of the Open Championship.
"I'm pretty motivated to get back to work," he added. "I feel I'm certainly playing better than I'm scoring and I'll look forward to working on it."
Mickelson was making his eighth attempt at completing the elusive career grand slam by winning the US Open - a tournament in which he has finished runner-up a record six times.
Instead, he missed the cut for just the fifth time in 31 appearances.