If this is on your '88 300 SEL, the tensioner is very similar to the 190E 2.3. Yes, it is important to loosen the large bolt in the middle first, but if you were loosening the belt it shouldn't matter...the belt would not loosen and you would realize that you forgot to loosen the large bolt.
Now if you were trying to tighten the belt, then yes, if you don't loosen the large bolt first, you could either break the Rod Assembly, or the Rod itself.

The tensioner is only damaged if the rubber bushing is torn or broken. If you by chance damaged the Rod Assembly, you can buy that separate.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible