WWE announced that they would reveal Brock Lesnar's Summer Slam opponent on SmackDown but there was not talk about Lesnar during the tapings.It appears that Brock's opponent would be revealed once SmackDown tapings air on Television.
However,Aftermatch TV on SportsNet in Cannada reported that Brock Lesnar's opponent for Summer Slam would be Randy Orton during their annalysis.But do note the fact that WWE never spoke a word about Lesnar's opponent till now so as of now this match is not confirmed and isn't official unless and until WWE makes this match official.