Is it the sole reason? No. Is it a factor? Most certainly. When you have an important piece of the team out you are forcing others to take on a bigger role. Guys like Manu, TD, Marco and Green are playing more minutes and are forced into larger offensive roles. Now could they handle it in a short period? Yes but over the long term it tends to get exploited, unless you have someone capable of filling that role. The Spurs don't really have one, which makes Kawhi 1 of the most (if not the most) important players on the team because there is a lack of replacement.
MPG since Kawhi has been out vs last season:
TD: 35.6 (32.3 excluding 3OT games) vs last year 29.2
Manu: 28.8 (26.5 excluding 3OT games) vs last year 22.8
Green: 35.6 (31.5 excluding 3OT games) vs last year 24.3
Marco: 32.7 (31.3 excluding 3OT games) vs last year 25.2