No option is perfect.
Ball is a true PG, a more than solid defender, still has remaining upside as he continues to fine tune the new mechanics on that ugly shot he had. At 6-6 he can guard 1 through 3, allowing the C's defense to switch and not be caught with glaring mismatches.
Is he perfect? No, he's not Luca. He's enough to improve team defense greatly, increase pace, and with Brown and Tatum as targets, his assists should greatly increase.
Fournier while playing well in the Olympics, did not overly impress in his short time with the Celtics. With Tatum and Brown in town he'll always be a 3rd option that brings little else to the team overall.