Tierney will miss the remainder of the season with a knee injury and 22-year-old Tavares was expected to come in and deputise.
Speaking on talkSPORT, Keown said: "Well, he [Nuno Tavares] probably isn't [good enough]. But if that was your boy out there, taken off after 25 minutes against [Nottingham] Forest.
"I would never have seen [Arsene] Wenger do that. He would have waited for the second half. This week he was taken off at half-time.
"We – Arsenal – have to find a solution to the left-back problem. I would like to see Saka go to left-back. I know that's radical but I'd rather see that than [Granit] Xhaka there who will get exposed.
"It's about your best 11. I think Saka can still do damage from left-back. [Nicolas] Pepe would go to the right and [Gabriel] Martinelli on the left. If you've got an issue with a player to such an extent that mistakes are being made down that side, then you can't play him [Tavares].
"But I don't think Xhaka is the answer. He certainly isn't. They'll be setting teams out to exploit that weakness. It won't be long before Xhaka is off the pitch and Arsenal are down to 10 men again. Play Saka there."