Dollar Tree Inc said on Monday Bob Sasser, the architect of the U.S. discount retailer's $9.1 billion acquisition of Family Dollar, will be replaced as chief executive by Gary Philbin, the company's enterprise president.

Sasser will become the executive chairman of Dollar Tree's board, effective immediately, the company said.

Philbin, 60, joined Dollar Tree as senior vice president of stores in 2001 and was named chief operating officer six years later. He was appointed enterprise president earlier this year with responsibilities for both the Dollar Tree and Family Dollar chains of stores.

"While this move was not telegraphed well in advance, we believe this will be a seamless transition as Philbin has very capably served the company ... and has helped Sasser lead the company for over 15 years," BTIG anlayst Alan Rifkin said in a note to clients.

Dollar Tree acquired Family Dollar in 2015 following a heated bidding war with rival Dollar General and became the biggest dollar store chain in the United States.

Dollar Tree's market value ballooned to about $19.8 billion from $2.3 billion since Sasser took the top job in 2004, the company said. Sasser joined the retailer as chief operating officer in 1999.

Shares of Dollar Tree were slightly higher at $83.59 in morning trading.