Complex sentence: In the 2010-11 NHL season, Thomas returned to form, making Rask once again the backup, but he won the Stanley Cup with Boston, becoming only the second Finnish goaltender to do so, after Chicago's Antti Niemi the previous year.
Split into simpler sentences: In the 2010-11 NHL season, Thomas returned to form, making Rask once again the backup. With the Bruins winning the Stanley Cup for Boston, Rask became only the second Finnish goaltender to do so, after Chicago's Antti Niemi the previous year.