Its not really the issue
IT on offense (once he get back to his old self) is great.
IT on defense was never great. KCP is the starting SG and despite being a dumbass occasionally, he is at least above average on defense (usually).
Lakers aren't really a great defensive team. They are decent, but there are liabilities all over defensively. Zo is solid (and so is Hart) but they are not lockdown defensive specialists at guard.
Isaiah just creates another liabilty defensively especially if his offense is off