"Chelsea Football Club and Bayer Leverkusen have reached an agreement for the transfer of Andre Schürrle subject to the completion of legal documentation and related matters, including personal terms and passing a medical," the Londoners said in a statement.
The transfer fee was not disclosed but media reports have put it at 18 million pounds.
German international Schürrle scored 11 Bundesliga goals last season.
The 22-year-old has enjoyed a rapid rise over the last three seasons, with his lightning-quick speed on the wing and excellent first touch making him a regular starter in the national team.
The signing of Schürrle could mean that fellow German Marko Marin, a disappointment since joining Chelsea from Werder Bremen 12 months ago or Victor Moses could be heading for the Stamford Bridge exit door.
Media reports have suggested Mourinho may move him on although the former Real Madrid, Inter Milan and Porto coach told his first news conference back at Stamford Bridge that every Chelsea player would be given a chance to prove his worth.
"I need to work with the players and not commit injustice," the 50-year-old Portuguese said on Monday.
"I need to give them a chance, be fair with them. After that we'll have time to make decisions."
Mata is now on Confederations Cup duty in Brazil with world and European champions Spain.
Mourinho, who was also in charge of Chelsea from 2004-07, will find out which opposition team he will start the season against when the Premier League announces the new fixture list on Wednesday.
Leverkusen have acted quickly to bring in Schürrle's replacement - 20-year-old South Korea international Son Heung-min.
