Transmission problems after engine swap

About a month and a half ago, I brought my car
(2005 Nissan Maxima) to a small auto shop owned
by my friend's father. I took the car in because the
timing chain system was bad. the mechanic
suggested that I replace the engine because there
were metal shavings in the oil pan. I was quoted for
about $1600 for the entire job which included a
used engine w/ about 84k miles on it and labor. I felt
it was a great deal and agreed for the work to be
done. When the work was done, I went to go pick it
up and was told by the mechanic to take it for a
spin. I drove the car for about 5 minutes then the
car started to jerk when I'd accelerate. Finally just a
few blocks from the shop, I tried to make a 3 point
turn and got stuck because I couldn't put the out
the car in gear. I would press the gas and the
engine would just rev. Luckily I was near the shop
and was able to call them to tow the car back. My
question is this. Do you guys think that the
transmission was damaged when the engine was
replaced and is the mechanic liable for this
damage? Mechanic did say that it is possible that
something in the tranny was touched or knocked
loose in the during the engine work. All I know is
that I had no issues with the transmission prior to
me bringing it to this shop. I am aware that 2005
Maximas are notorious for having faulty
transmissions and timing chain systems. I'm also
aware that there is was a class action lawsuit
involving this year/make/model. Unfortunately the
mechanic is too and seems to not be taking
responsibility for this damage. What do you guys
think? Did the the mechanic damage the
transmission when he removed and installed a new
engine? Or was my tranny already messed up and
this is all just a case of bad timing??