I would buy the rustiness excuse if Tek was not known for being blunder prone way before his retirement
If he didn't openly say he has lost the motivation for this sport which is why he retired in the first place. For me at least, motivation and willingness to fight for the team and win is very important for every goalkeeper. If Tek lacks that, it is questionable how much he can contribute.
I will however support whatever Flick is doing because, like many have pointed out, he watches them in training every day so probably knows better than us. Maybe he values shot stopping over sweeping or thinks it might be more helpful for the team at the moment, I don't know.