The senior guard now has played in four games since his return and has yet to hit a jump shot while going 2 of 8 from the free-throw line – including a 1 of 5 effort in Michigan State's 53-46 loss to Illinois on Saturday.
"Keith Appling is a major problem right now because he can't do what he could do," Izzo said. "They're not backing off of him and there's nowhere for him to go."
The senior, who had five games with 20 or more points and seven or more assists before missing time with a right wrist injury, is averaging just 3.5 points per game in the four since his return. Michigan State is 1-3 in those games.
"Compared to how he was playing at the beginning of Big Ten season, he's not there," Izzo said. "Some of that is not confidence, he just can't do it."
Appling seconded that, saying it isn't a lack of confidence, but just how the wrist feels.
"It's all a matter of getting back comfortable to doing the things that I was before," he said. "I feel like I kind of babied my wrist and my hand for most part of the season.
"Now that it's feeling a lot better, I just gotta make that adjustment."
Izzo said Appling is getting better and has been working on his shot, but noted it is going to take some time. Following Saturday's loss, Appling said he is in the process of readjusting his shot after learning to shoot with his ailing wrist.
"I'm so used to having to shoot a certain way because of my wrist," he said. "Now that it's feeling better, I have to make that adjustment because it's how I shot for so long."
He hopes he can get back to the way he was shooting earlier in the year, but with just two regular-season games remaining, time is short.
"I'm working every day to get back there," he said. "I've just gotta continue to do that and see where it gets me."