The 26-year-old centre-back recently rejected an offer of a new deal from Dortmund and will enter the final year of his contract with the club at the end of the season.
He has started 16 of the 17 Bundesliga games that he has been available to play this season but his future at the club is not iron clad anymore.
There are suggestions Dortmund are likely to cash in on him and Manchester United are claimed to be interested in signing him.
But according to German broadcaster Sport1, they are not the only Premier League club who are keeping tabs on the Swiss defender.
His contractual situation is being watched by several other clubs who are prepared to pounce in the summer.
Dortmund are expected to let the player go if they receive a fee in the region of the €30m mark at the end of the season.
The Bundesliga giants have already agreed on a pre-contract to sign Bayern Munich defender Niklas Sule on a free transfer in the summer.
