The Premier League trio ran the rule over the £12m-rated Turkey centre-back in Leverkusen's Champions League win over Zenit last week.
Two-footed Toprak is renowned for his speed, reading of the game and calm leadership style.
And his displays have caught the attention of United, Liverpool and Spurs who are all on the lookout for defensive reinforcements.
Toprak's current deal with Leverkusen runs until 2018.
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