Marcus was horrendous at the end of the game. However, the issue remains Jaylen Brown. He can play like an All Star for the first three quarters, but is non-existent in the fourth. That is the issue Brad, Ime, and Jaylen need to figure out.
Tatum is a very capable closer. However, since Brown essentially takes himself out of the game in the fourth, he no longer becomes a threat, which puts more pressure on Tatum and the rest of the offense.
As much as I like Lillard, he is no longer an answer at this point in his career. Bradley Beal might be, but I don't think it will happen, with the main issue being on defense and his lack of ability to switch. John Collins would be great, but I don't know how you trade for him without giving up Brown or Smart. For me, the answer is figuring out how to get Jaylen to be more effective in the fourth, and specifically at the end of games.