I am flying from Boston to Amsterdam via Manchester where I have only 75 minutes between the two flights. I need to change terminals and I guess I need to pass Security since I am flying from outside the EU.
Will this be enough time?
If you're booked on a single ticket, and it wouldn't be awful if you ended up being a few hours late in Amsterdam, then I wouldn't worry. There are lots of flights from Manchester to Amsterdam (around 10-15 direct per day), and you'll just be put on a later one.
In terms of the transfer experience, here's one person's experience of a 1h15m international to domestic airside connection at Manchester - it took 40 minutes from arrival to being at the gate for the next flight. It also seems to be a worst-case in terms of time spent waiting for the bus and includes a passport check you wouldn't have to go through: http://philatravelgirl.com/connecting_in_manchester_phl-man_airport_/
The link also implies that you get a separate security area so as long as you're quick off the bus to the connections centre you shouldn't have to wait for too long.