Sri Lanka did not say whether wicketkeeper-batsman Mendis would be available for the ODIs.
Fast bowler Binura Fernando is also undergoing rehabilitation for injury.
"Mendis suffered a right hamstring injury while Fernando sustained a back injury," SLC said in a statement.
Batsman Pavan Rathnayake and fast bowler Dilshan Madushanka replace them.
The first T20 is on September 15 at Southampton, with the second two days later in Cardiff. The final T20 is on September 19 at Manchester.
The ODI series begins on September 22 at Chester-le-Street, followed by matches at Headingley on September 24 and The Oval on September 27.
Revised T20 squad: Charith Asalanka (capt), Pathum Nissanka, Kamil Mishara, Lahiru Udara, Kamindu Mendis, Janith Liyanage, Dasun Shanaka, Dunith Wellalage, Wanindu Hasaranga, Maheesh Theekshana, Tharindu Rathnayake, Dushmantha Chameera, Eshan Malinga, Nuwan Thushara, Pavan Rathnayake and Dilshan Madushanka.
