Cutting estimates for Electronic Arts "highlights risks of a back-end loaded year, crowded holiday title slate and continued Fortnite pressure" on Activision, the note said.
Activision Blizzard shares closed down 2.7 percent Friday, while EA shares fell 2.2 percent.
The analyst lowered his price target to $126 from $159 for EA shares. He also reduced his target for Activision Blizzard to $77 from $84.
Post cited how "Fortnite" continues expand its user base on a monthly basis, which may negatively affect the large publishers' game sales later this year. Increased competition in the first-person shooter game market, as shown by the "Battlefield V" issues, will likely pressure publishers' high valuation multiples, he said.
Activision Blizzard shares are up 17 percent this year through Thursday, while EA's stock is up 10.4 percent. In the same time period, the S&P 500 rose 8.5 percent.
But a crowded fourth quarter "may lead to spillover challenges in early 2019 as publishers vie for gamer engagement to drive live service initiatives," he said.
EA declined to comment on the analyst report. Activision Blizzard did not immediately respond to a request for comment.