The context seems significant in this case. I think the lines mean something like:

だが、そもそも俺はまともに巨人化できるのか？
But am I even able to properly transform into a 巨人 to begin with?

ライナーができないと言うように(なるかもしれない)
Things will likely go as Reiner said, which means I will probably fail (if I try)

もしくは
or

ライナーでさえここで休まざるを得ない現状を鑑みるに
Judging from the fact that even Reiner has to take a break here

下手に体力のない巨人を生み出せばほかの巨人にやられちまうってことか？
I will probably end up eaten other 巨人 if I bungle it and transform into a weak 巨人

鑑みるに is explained in this answer:

Grammar behind 言うには

And ライナーでさえここで休まざるを得ない現状を鑑みるに apparently connects with the following line, which is why context is important here.

The context seems significant in this case. I think the lines mean something like:

だが、そもそも俺はまともに巨人化できるのか？
But am I even able to properly transform into a 巨人 to begin with?

ライナーができないと言うように(なるかもしれない)
Things will likely go as Reiner said, which means I will probably fail (if I try)

もしくは
or

ライナーでさえここで休まざるを得ない現状を鑑みるに
Judging from the fact that even Reiner has to take a break here

下手に体力のない巨人を生み出せばほかの巨人にやられちまうってことか？
I will probably end up eaten by other 巨人 if I bungle it and transform into a weak 巨人

鑑みるに is explained in this answer:

Grammar behind 言うには

And ライナーでさえここで休まざるを得ない現状を鑑みるに apparently connects with the following line, which is why context is important here.